This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
FO
Flare Details for Petroleum,
Petrochemical and Natural
Gas Industries
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API STANDARD 537
THIRD EDITION, MARCH 2017
20180126 RW: Updated file to reflect Third Edition publication
USE THIS FILE FOR 3rd EDITION, 1st ADDENDUM
DEVELOPMENT
Draft Development Record:
20180213 RW: Added Annex G with sections migrated from API 521 Sixth Edition.
Bibliographic references in Annex G have not been updated.
20180126 RW: Updated file to reflect Third Edition publication
API Staff NOTE: Draft for Spring 2018 Technical Ballot to assist the RP 537 Task Group in
further developing the draft. This ballot is not for approval for publication.
i
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publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
Foreword
FO
Nothing contained in any API publication is to be construed as granting any right, by implication or otherwise, for
the manufacture, sale, or use of any method, apparatus, or product covered by letters patent. Neither should
anything contained in the publication be construed as insuring anyone against liability for infringement of letters
patents.
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Shall: As used in a standard, “shall” denotes a minimum requirement in order to conform to the specification.
Should: As used in a standard, “should” denotes a recommendation or that which is advised, but not required in
order to conform to the specification.
May: As used in a standard, “may” denotes a course of action permissible within the limits of a standard.
Can: As used in a standard, “can” denotes a statement of possibility or capability.
This document was produced under API standardization procedures that ensure appropriate notification and
participation in the developmental process and is designated as an API standard. Questions concerning the
interpretation of the content of this publication or comments and questions concerning the procedures under
which this publication was developed should be directed in writing to the Director of Standards, American
Petroleum Institute, 1220 L Street, NW, Washington, D.C. 20005. Requests for permission to reproduce or
translate all or any part of the material published herein should also be addressed to the director.
Generally, API standards are reviewed and revised, reaffirmed, or withdrawn at least every five years. A one-time
extension of up to two years may be added to this review cycle. Status of the publication can be ascertained from
the API Standards Department, telephone (202) 682-8000. A catalog of API publications and materials is
published annually and updated quarterly by API, 1220 L Street, NW, Washington, D.C. 20005.
Suggested revisions are invited and should be submitted to the Standards Department, API, 1220 L Street, NW,
Washington, D.C. 20005, standards@api.org.
ii
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
Contents
Scope ...................................................................................................................................................... 1
2
Normative References ........................................................................................................................... 1
3
Terms and Definitions ........................................................................................................................... 2
4
4.1
4.2
4.3
4.4
4.5
4.6
4.7
4.8
4.9
4.10
4.11
Design ..................................................................................................................................................... 8
General ................................................................................................................................................... 8
System Design ....................................................................................................................................... 9
Process Definition ................................................................................................................................. 9
Types of Flares .................................................................................................................................... 10
Flare-burners........................................................................................................................................ 10
Mechanical Design .............................................................................................................................. 11
Pilots ..................................................................................................................................................... 12
Pilot-ignition Systems ......................................................................................................................... 13
Pilot-flame Detection ........................................................................................................................... 13
Piping .................................................................................................................................................... 14
Auxiliary Components ........................................................................................................................ 14
5
5.1
5.2
5.3
5.4
5.5
5.6
5.7
5.8
5.9
5.10
Mechanical Details — Elevated Flares .............................................................................................. 15
Mechanical Design — Design Loads ................................................................................................. 15
Design Details ...................................................................................................................................... 19
Flanges ................................................................................................................................................. 20
Materials of Construction ................................................................................................................... 21
Welding ................................................................................................................................................. 22
Inspection ............................................................................................................................................. 22
Surface Preparation and Protection .................................................................................................. 23
Attachments ......................................................................................................................................... 23
Aircraft Warning Lighting ................................................................................................................... 23
Platforms and Ladders ........................................................................................................................ 23
6
6.1
6.2
6.3
6.4
6.5
6.6
Mechanical Details — Enclosed-flame Flares .................................................................................. 24
Combustion Chamber ......................................................................................................................... 24
Burners ................................................................................................................................................. 24
Burner Piping ....................................................................................................................................... 25
Pilots ..................................................................................................................................................... 25
Wind Fence........................................................................................................................................... 26
Radiation Shielding ............................................................................................................................. 26
Annex
A.1
A.1.1
A.1.2
A.1.3
A.1.4
A.1.5
A.1.6
A.1.7
A.1.8
A.1.9
A.2
A.2.1
A.2.2
A.2.3
A.2.4
A.2.5
A.2.6
A.2.7
A.2.8
A.3
A.3.1
A (informative) Flare Equipment Overview ....................................................................................... 27
Types of Flares and Components ...................................................................................................... 27
General ................................................................................................................................................. 27
Elevated ................................................................................................................................................ 28
Horizontal ............................................................................................................................................. 34
Enclosed-flame Flares ........................................................................................................................ 35
Single- and Multi-burner ..................................................................................................................... 36
Smokeless and Non-smokeless Flares ............................................................................................. 37
Endothermic (Fuel-gas-assisted) Flares ........................................................................................... 40
Major Components .............................................................................................................................. 40
Ladders and Platforms ........................................................................................................................ 41
Flare-burner.......................................................................................................................................... 42
Purpose ................................................................................................................................................ 42
Unassisted Pipe Flare ......................................................................................................................... 42
Steam-assisted Pipe Flare .................................................................................................................. 43
Pipe Flares with Internal Steam/Air Eductor Tubes ......................................................................... 44
Air-assisted Smokeless Flares........................................................................................................... 45
High-pressure Smokeless Flares ....................................................................................................... 46
Mechanical Details of Flare-burners .................................................................................................. 48
Operations ............................................................................................................................................ 50
Pilots ..................................................................................................................................................... 51
Purpose ................................................................................................................................................ 51
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1
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This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
General Description ............................................................................................................................ 51
Mechanical Details .............................................................................................................................. 52
Operation ............................................................................................................................................. 56
Maintenance ........................................................................................................................................ 56
Troubleshooting.................................................................................................................................. 56
Ignition Equipment ............................................................................................................................. 57
Purpose ................................................................................................................................................ 57
General Description ............................................................................................................................ 57
Mechanical Details .............................................................................................................................. 58
Operation ............................................................................................................................................. 62
Maintenance ........................................................................................................................................ 64
Troubleshooting.................................................................................................................................. 64
Flame-detection Equipment ............................................................................................................... 67
Purpose ................................................................................................................................................ 67
General Description ............................................................................................................................ 67
Mechanical Details .............................................................................................................................. 68
Optical Systems .................................................................................................................................. 68
Acoustic Systems ............................................................................................................................... 68
Operation ............................................................................................................................................. 68
Maintenance ........................................................................................................................................ 69
Troubleshooting.................................................................................................................................. 69
Verification Test .................................................................................................................................. 71
General ................................................................................................................................................. 71
Setup .................................................................................................................................................... 71
Testing ................................................................................................................................................. 71
Purge-gas Conservation Seals .......................................................................................................... 71
Purpose ................................................................................................................................................ 71
General Description ............................................................................................................................ 72
Mechanical Details .............................................................................................................................. 72
Maintenance ........................................................................................................................................ 72
Troubleshooting.................................................................................................................................. 72
Knockout Drums and Liquid Seals ................................................................................................... 74
Knockout Drum ................................................................................................................................... 74
Liquid Seals ......................................................................................................................................... 74
Blowers and Drivers ........................................................................................................................... 75
Purpose ................................................................................................................................................ 75
General Description ............................................................................................................................ 75
Mechanical Details .............................................................................................................................. 75
Operations ........................................................................................................................................... 77
Maintenance ........................................................................................................................................ 77
Troubleshooting.................................................................................................................................. 78
Blower Staging and Control Equipment ........................................................................................... 79
Purpose ................................................................................................................................................ 79
General Description ............................................................................................................................ 79
Mechanical Details .............................................................................................................................. 79
Operations ........................................................................................................................................... 81
Maintenance ........................................................................................................................................ 81
Troubleshooting of Blower Staging and Control Systems ............................................................. 82
Pressure-staging Equipment ............................................................................................................. 82
Purpose ................................................................................................................................................ 82
General Description ............................................................................................................................ 83
Mechanical Details .............................................................................................................................. 84
Operations ........................................................................................................................................... 85
Maintenance ........................................................................................................................................ 85
Troubleshooting.................................................................................................................................. 85
Flow and Pressure Sensing Equipment ........................................................................................... 86
Purpose ................................................................................................................................................ 86
General Description ............................................................................................................................ 86
Mechanical Description ...................................................................................................................... 87
Operations ........................................................................................................................................... 87
Maintenance ........................................................................................................................................ 87
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A.3.2
A.3.3
A.3.4
A.3.5
A.3.6
A.4
A.4.1
A.4.2
A.4.3
A.4.4
A.4.5
A.6
A.5
A.5.1
A.5.2
A.5.3
A.5.4
A.5.5
A.5.6
A.5.7
A.5.8
A.6
A.6.1
A.6.2
A.6.3
A.7
4.7.1
4.7.2
4.7.3
A.7.4
A.7.5
A.8
A.8.1
A.8.2
A.9
A.9.1
A.9.2
A.9.3
A.9.4
A.9.5
A.9.6
A.10
A.10.1
A.10.2
A.10.3
A.10.4
A.10.5
A.10.6
A.11
A.11.1
A.11.2
A.11.3
A.11.4
A.11.5
A.11.6
A.12
A.12.1
A.12.2
A.12.3
A.12.4
A.12.5
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
A.12.6 Troubleshooting .................................................................................................................................. 87
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Annex B (informative) Components of Multi-burner Staged Flare Equipment ......................................... 88
B.1
Burners ................................................................................................................................................. 88
B.2
Pilots ..................................................................................................................................................... 89
B.3
Ignition Equipment .............................................................................................................................. 89
B.4
Flame-detection Equipment ............................................................................................................... 89
B.5
Buoyancy and Velocity Seals ............................................................................................................. 89
B.6
Manifolds .............................................................................................................................................. 89
B.7
Operations ............................................................................................................................................ 89
B.8
Maintenance ......................................................................................................................................... 90
B.9
Troubleshooting .................................................................................................................................. 90
Annex C (informative) Enclosed-flame Flares .............................................................................................. 91
C.1
Purpose ................................................................................................................................................ 91
C.2
General Description ............................................................................................................................ 91
C.2.1 Overview ............................................................................................................................................... 91
C.2.2 Combustion-chamber Size and Shape .............................................................................................. 92
C.2.3 Mechanical Considerations ................................................................................................................ 94
C.2.4 Burners ................................................................................................................................................. 94
C.2.5 Burner Piping ....................................................................................................................................... 94
C.2.6 Pilots ..................................................................................................................................................... 94
C.2.7 Air and Flue-gas Flows ....................................................................................................................... 95
C.2.8 Wind Fences......................................................................................................................................... 96
C.2.9 Operational and Safety Controls........................................................................................................ 98
C.2.10 Enclosed-flame Flare Applications .................................................................................................... 98
C.3
Operating Considerations for Enclosed-flame Flares ..................................................................... 99
C.3.1 Visible Flames ...................................................................................................................................... 99
C.3.2 Noise and Vibration ............................................................................................................................. 99
C.3.3 Refractory Failure .............................................................................................................................. 100
C.3.4 Pre-commissioning ........................................................................................................................... 100
C.3.5 Commissioning .................................................................................................................................. 100
C.3.6 Normal Operations ............................................................................................................................ 101
C.3.6.1 Multiple-flare Operations .................................................................................................................. 101
C.4
Maintenance ....................................................................................................................................... 102
C.5
Troubleshooting ................................................................................................................................ 103
Annex D (informative) Offshore Flare Systems .......................................................................................... 105
D.1
Design Considerations for Offshore Flare Systems ...................................................................... 105
D.1.1 Location Considerations .................................................................................................................. 105
D.1.2 Structural Type Considerations ....................................................................................................... 106
D.1.3 Height Considerations ...................................................................................................................... 107
D.1.4 Thermal Radiation Shielding Considerations ................................................................................. 107
D.1.5 Structural Considerations ................................................................................................................ 108
D.2
Design Considerations for Flare-burners ....................................................................................... 108
D.2.1 General Considerations .................................................................................................................... 108
D.2.2 Firing Angle ........................................................................................................................................ 109
D.2.3 Materials of Construction ................................................................................................................. 109
D.3
Flare Pilots, Ignition and Purging Systems .................................................................................... 110
D.3.1 Flare Pilots ......................................................................................................................................... 110
D.3.2 Flare Pilot Ignition System ............................................................................................................... 110
D.3.3 Flare-burner Direct Ignition .............................................................................................................. 110
D.3.4 Purge Gas ........................................................................................................................................... 111
D.4
Maintenance and Inspection Considerations ................................................................................. 111
D.5
Flare-burner Inspection Frequency ................................................................................................. 113
D.6
Considerations for Offshore Applications ...................................................................................... 113
D.6.1 Vertical Flare Stacks ......................................................................................................................... 113
D.6.2 Flare Boom Stacks ............................................................................................................................ 113
D.6.3 Separate Flare Structure with a Bridge or Subsea Connection to the Main Platform ................ 113
D.6.4 Structural ............................................................................................................................................ 113
D.6.5 Materials of Construction ................................................................................................................. 113
D.7
Reliability Analysis ............................................................................................................................ 113
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This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
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Annex E (informative) Instructions for Flare Data Sheets ........................................................................ 114
E.1
Introduction ....................................................................................................................................... 114
E.2
General Information Forms — Instructions ................................................................................... 115
E.2.1 Form General 1 .................................................................................................................................. 115
E.2.2 Form General 2 .................................................................................................................................. 116
E.2.3 Form General 3 .................................................................................................................................. 117
E.2.4 Form General 4 .................................................................................................................................. 119
E.2.5 Form General 5 .................................................................................................................................. 119
E.2.6 Form General 6 .................................................................................................................................. 120
E.2.7 Form General 7 .................................................................................................................................. 120
E.3
Elevated-flare Forms — Instructions .............................................................................................. 120
E.3.1 Form Elevated 1 ................................................................................................................................ 120
E.3.2 Form Elevated 2 ................................................................................................................................ 121
E.3.3 Form Elevated 4 ................................................................................................................................ 121
E.3.4 Form Elevated 5 ................................................................................................................................ 122
E.4
Enclosed-flare Forms — Instructions ............................................................................................. 122
E.4.1 Form Enclosed 1 ............................................................................................................................... 122
E.4.2 Form Enclosed 2 ............................................................................................................................... 123
E.4.3 Form Enclosed 3 ............................................................................................................................... 123
E.4.4 Form Enclosed 4 ............................................................................................................................... 124
E.4.5 Form Enclosed 5 ............................................................................................................................... 124
Annex F (informative) Flare Data Sheets .................................................................................................... 125
F.1
SI Units ............................................................................................................................................... 125
F.2
USC Units .......................................................................................................................................... 143
Annex G (informative) API Standard 521 Sixth Edition Migration ........................................................... 161
G.1
General ............................................................................................................................................... 161
G.2
Combustion Properties .................................................................................................................... 162
G.2.1 Smoke (5.7.2.2) .................................................................................................................................. 162
G.2.2 Flame Stability (5.7.2.4) .................................................................................................................... 162
G.3
Combustion Methods ....................................................................................................................... 163
G.3.1 Flares with Smoke Suppression (5.7.3.2) ....................................................................................... 163
G.3.2 Steam Requirements for Steam Assisted Flares (5.7.3.2.2) ......................................................... 163
G.3.3 Degree of Smokelessness (5.7.3.2.3) .............................................................................................. 165
G.3.4 Steam Injection (5.7.3.2.4) ................................................................................................................ 165
G.3.5 High-pressure Air (5.7.3.2.5) ............................................................................................................ 166
G.3.6 High-pressure Water (5.7.3.2.6) ....................................................................................................... 167
G.3.7 Low-pressure Forced-air System (5.7.3.2.7) .................................................................................. 168
G.3.8 High-pressure Flaring (5.7.3.2.8) ..................................................................................................... 169
G.3.9 High-pressure Flaring (5.7.3.2.9) ..................................................................................................... 170
G.3.10 Control of Fluid Injection for Smoke Suppression (5.7.3.2.10) .................................................... 170
G.3.11 Noise Caused by Smokeless Flaring (5.7.3.2.11) .......................................................................... 171
G.3.12 Noise Caused by Smokeless Flaring (5.7.3.3) ............................................................................... 172
G.3.13 Flaring of Gases with Low Heating Value (5.7.3.4) ........................................................................ 172
G.4
Flare System Design......................................................................................................................... 173
G.4.1 Ground Flares (5.7.4.1) ..................................................................................................................... 173
G.4.2 Elevated Flares (5.7.4.2) ................................................................................................................... 173
G.4.3 Unassisted Pipe Flares (5.7.4.1) ...................................................................................................... 174
G.4.4 Burn Pits (5.7.4.4).............................................................................................................................. 175
G.5
Sizing.................................................................................................................................................. 175
G.5.1 General (5.7.5.1) ................................................................................................................................ 175
G.5.2 Flare Riser Diameter (5.7.5.2) .......................................................................................................... 176
G.5.3 Flare Stack Height (G.5.3) ................................................................................................................ 176
G.5.4 Flare Burner Pressure Drop (5.7.5.4) .............................................................................................. 177
G.6
Purging .............................................................................................................................................. 177
G.6.1 General (5.7.6.1) ................................................................................................................................ 177
G.6.2 Air Infiltration/Continuous Purging Requirements for Stacks Without a Purge Reduction Seal
(5.7.6.2) ............................................................................................................................................... 179
G.7
Ignition of Flare Gases ..................................................................................................................... 181
G.7.1 General (5.7.7.1) ................................................................................................................................ 181
G.7.2 Pilot Fuel Gas Supply (5.7.7.2) ........................................................................................................ 181
vi
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
G.7.3
G.7.4
Pilot Monitoring (5.7.7.3) ................................................................................................................... 182
Retractable Thermocouples (new) ................................................................................................... 182
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This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
Introduction
FO
A flare is a critical mechanical component of a complete system design intended for the safe, reliable and efficient
discharge and combustion of hydrocarbons from pressure-relieving and vapor-depressurizing systems.
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The high-level safety and operating goals of a flare are summarized as follows:
to provide safe, reliable and efficient discharge and combustion of hydrocarbons with a high combustion
efficiency;
to ensure that the discharged hydrocarbons burn with stable combustion over the entire defined operating
range;
to ensure a continuity of the flare flame under severe weather conditions;
to ensure that ground level concentrations of specified compounds do not exceed environmental limits;
to ensure that the back pressure does not exceed the maximum allowable;
to ensure that velocity throughout the flare piping and the flare-burner does not exceed the maximum
specified;
to ensure that the opacity limit at the smokeless flow rate range does not exceed that defined;
to ensure that the flare radiation intensity does not exceed the maximum allowable; and
to ensure that noise levels do not to exceed the maximum permissible.
For new designs, the development of a design can be advanced using the guidance and examples of good
engineering practice that are identified in this standard.
A flare design basis is developed in consideration of the performance expectations, the functional requirements
and mechanical details required to fulfill the safety and operating goals established for each application. Section 4
provides the basis for design and functional requirements related to the primary components critical to fulfilling
these safety and operating goals. Section 5 and Section 6 provide requirements more specific to the arrangement
and mechanical details of design.
The functional requirements in this standard are supported by the technical guidance provided in Annex A,
Annex B, Annex C and Annex D. The technical guidance provided in the informative annexes addresses
alternative designs or techniques and provides good practices on the basis of which, through sound engineering
judgment, the practitioner can make appropriate design decisions and selections.
Data sheets are provided in Annex F in order to properly communicate and preserve the finalized basis of design
and requirements. Annex E provides instructions for completing the flare data sheets in Annex F.
Users of this standard should be aware that further or differing requirements may be needed for individual
applications. This standard is not intended to inhibit a vendor from offering, or the purchaser from accepting,
alternative equipment or engineering solutions for the individual application. This may be particularly applicable
where there is innovative or developing technology. Where an alternative is offered, the vendor should identify
any variations from this Standard and provide details.
The SI system of units is used in this standard. Where practical, US Customary units are included in brackets for
information.
viii
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
A bullet () at the beginning of a clause or sub-clause indicates that either a decision is required or further
information is to be provided by the purchaser. This information should be indicated on data sheets (see
examples in Annex F) or stated in the inquiry or purchase order.
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This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
FO
Flare Details for Petroleum, Petrochemical and Natural Gas
Industries
Scope
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1
This standard specifies requirements and provides guidance for the selection, design, specification, operation and
maintenance of flares and related combustion and mechanical components used in pressure-relieving and vapordepressurizing systems for petroleum, petrochemical and natural gas industries. While this standard is primarily
intended for onshore facilities, guidance related to offshore applications is included.
Annex A, Annex B, Annex C and Annex D provide further guidance and best practices for the selection,
specification and mechanical details for flares and on the design, operation and maintenance of flare combustion
and related equipment.
Annex E explains how to use the data sheets provided in Annex F; it is intended that these data sheets be used to
communicate and record design information.
2
Normative References
The following referenced documents are indispensable for the application of this document. For dated references,
only the edition cited applies. For undated references, the latest edition of the referenced document (including any
amendments) applies.
API Recommended Practice 2A WSD:2000, Recommended Practice for Planning, Designing and Constructing
Fixed Offshore Platforms — Working Stress Design
API Standard 521, Pressure-Relieving and Depressuring Systems
API Standard 560, Fired Heaters for General Refinery Service
ASME B16.51, Pipe Flanges and Flanged Fitting
ASME STS-1, Steel Stacks
ASTM A123/A123M2, Standard Specification for Zinc (Hot-Dip Galvanized) Coatings on Iron and Steel Products
ASTM A143/A143M, Standard Practice for Safeguarding Against Embrittlement of Hot-Dip Galvanized Structural
Steel Products and Procedure for Detecting Embrittlement
ASTM A153/A153M, Standard Specification for Zinc Coating (Hot-Dip) on Iron and Steel Hardware
ASTM A384/A384M, Standard Practice for Safeguarding Against Warpage and Distortion During Hot-Dip
Galvanizing of Steel Assemblies
ASTM A385/385M, Standard Practice for Providing High-Quality Zinc Coatings (Hot-Dip)
ASTM A475-03, Standard Specification for Zinc-Coated Steel Wire Strand
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2
American Society of Mechanical Engineers, 3 Park Avenue, New York, NY, 10016-5990, www.asme.org.
American Society for Testing and Materials, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, www.astm.org.
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2
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
ASTM A586-04a, Standard Specification for Zinc-Coated Parallel and Helical Steel Wire Structural Strand
ASTM B633, Standard Specification for Electrodeposited Coatings of Zinc on Iron and Steel
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EN 1092-1:2007 3 , Flanges and their joints — Circular flanges for pipes, valves, fittings and accessories, PN
designated — Part 1: Steel flanges
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EN 10264-2:2002, Steel wire and wire products — Steel wire for ropes - Part 2: Cold drawn non alloy steel wire
for ropes for general applications
EN 12385-10, Steel wire ropes — Safety — Part 10: Spiral ropes for general structural applications
ISO 2408:20044, Steel wire ropes for general purposes — Minimum requirements
NACE MR01035, Petroleum, petrochemical and natural gas industries — Metallic materials resistant to sulfide
stress cracking in corrosive petroleum refining environments
NACE MR0175, Petroleum and natural gas industries – Materials for use in H2S-containing environments in oil
and gas production, Parts 1, 2 and 3
SSPC SP 6/NACE No. 36, Commercial Blast Cleaning
3
Terms and Definitions
For the purposes of this document, the following terms and definitions apply.
3.1
air seal
purge reduction device
Device used to reduce the amount of purge gas needed to protect against air infiltration into the flare stack.
NOTE
Examples include buoyancy seal, orifice seal and velocity seal.
3.2
assist gas
Fuel gas that is added to relief gas prior to the flare-burner or at the point of combustion in order to raise the
heating value
NOTE 1
Also refer to enrichment (3.23) and supplemental gas (3.60).
NOTE 2
In some designs, the assist gas can increase turbulence for improved combustion.
3.3
back blowing
Procedure by which the dry air seal drain line is blown-back from the base of the drain into the buoyancy seal to
ensure the line is clear.
3.4
blowoff
Loss of a stable flame where the flame is lifted above the burner that occurs when the fuel velocity exceeds the
flame velocity.
3
4
5
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CEN European Committee for Standardization, 36 rue de Stassart, B-1050 Brussels, Belgium, www.cen.eu.
International Organization for Standardization, 1, ch. de la Voie-Creuse, Case postale 56, CH-1211, Geneva, Switzerland,
www.iso.org.
NACE International (formerly the National Association of Corrosion Engineers), 1440 South Creek Drive, Houston, Texas
77218-8340, www.nace.org.
The Society for Protective Coatings, 40 24th Street, 6th Floor, Pittsburgh, Pennsylvania 15222-4643, www.sspc.org.
3
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
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3.5
buoyancy seal
diffusion seal
density seal
Dry vapor seal that minimizes the required purge gas needed to protect from air infiltration.
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NOTE
The buoyancy seal functions by trapping a volume of light gas in an internal inverted compartment that prevents
air from displacing buoyant light gas in the flare.
3.6
burnback
Internal burning within the burner.
NOTE
Burnback can result from air backing down the flare-burner at purge or low flaring rates.
3.7
burn-pit flare
Open excavation, normally equipped with a horizontal flare-burner that can handle liquid as well as gaseous
hydrocarbons.
3.8
burning velocity
flame velocity
Speed at which a flame front travels into an unburned combustible mixture.
3.9
Coanda flare
Flare-burner that is designed to employ the aerodynamic effect where moving fluids follow a curved or inclined
surface over which they flow.
NOTE
Flares of this type generally use steam or pressure to achieve smokeless performance.
3.10
combustion air
Air required to combust the flare gases.
3.11
combustion efficiency
Percentage of the combustible fluid totally oxidized by the burner.
NOTE
In the case of hydrocarbons, combustion efficiency is the mass percent of carbon in the original fluid that oxidizes
completely to CO2.
3.12
condensable gas
Vapor that can condense at the temperature and pressure expected in a flare header during or after a flaring
event.
3.13
cryogenic service
Systems that may be called upon to handle waste gas below 29 °C (20 °F).
3.14
derrick support
Lattice based support system for the elevated flare stack, normally used for very tall flares or when plot space is
limited.
NOTE
Various derrick-supported arrangements are available: a fixed system has its stack permanently supported to the
derrick; a demountable derrick has multiple riser sections that are designed such that they can be lowered and removed to
4
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
permit lowering of the flare-burner to grade; a demountable derrick with one fixed riser provides for a single-piece design that
can be lowered to grade as a single component.
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3.15
design flare capacity
Maximum design flow to the flare.
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NOTE
The design flare capacity is normally expressed in kilograms per hour (pounds per hour) of a specific composition,
temperature and pressure.
3.16
destruction efficiency
Mass percent of the original combustible vapor that is no longer present in the combustion products.
NOTE
In the case of a hydrocarbon, destruction efficiency is the mass percentage of carbon in the fluid vapor that
oxidizes to CO or CO2.
3.17
detached stable flame
Flame that is not in contact with the flare-burner itself, but burns with a stable flame-front in the vicinity of the flareburner.
3.18
direct ignition
Ignition of a flare-burner by a high-energy (sparking) source other than by a pilot flame.
3.19
dispersion
Scattering of the products of combustion over a wide area to reduce ground-level concentrations of the
combustion products.
3.20
elevated flare
Flare where the burner is raised high above ground level to reduce radiation intensity and to aid in dispersion.
3.21
enclosed flare
Flare enclosure with one or more burners arranged in such a manner that the flame is not directly visible from
grade.
3.22
endothermic flare
Flare that utilizes supplemental energy, usually assist or enrichment gas, to maintain the combustion reaction.
3.23
enrichment
Process of adding assist gas to the relief gas to increase the heating value.
3.24
excess air
Air provided to a flame in excess of stoichiometric requirements.
3.25
exit velocity
Velocity at which the fluid exits the burner.
NOTE
The exit velocity is usually expressed as meters per second (feet per second) or as the Mach number for the fluid.
5
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
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3.26
flame detection system
flame monitor
System that verifies a flame is present.
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3.27
flame-front generator
Device used for lighting a pilot by means of a flame front.
NOTE
A combustible gas-air mixture is created and allowed to fill an ignition line connecting the flame-front generator
and the pilot. Igniting the mixture allows the flame front to travel through the ignition line to the pilot.
3.28
flame retention device
Physical device meant to prevent flame blow-off from a flare-burner.
3.29
flare
Device or system used to safely dispose of relief fluids in an environmentally compliant manner through the use of
combustion.
3.30
flare-burner
flare-tip
tip
Part of the flare where fuel and air are mixed at velocities, turbulence and concentration required to establish and
maintain proper ignition and stable combustion.
NOTE
The name “flare-burner” is considered more appropriate than “flare tip,” given the engineered nature of design and
inclusion of measures for flame stabilization, most often of proprietary design.
3.31
flare header
Piping system that collects and delivers the relief gases to the flare.
3.32
flare stack
flare boom
flare tower
riser
Mechanical device upon which an elevated flare-burner is mounted see also 3.55
3.33
flashback
Phenomenon occurring in a flammable mixture of air and gas when the local velocity of the combustible mixture
becomes less than the flame velocity, causing the flame to travel back to the point of mixture.
3.34
ground flare
Non-elevated flare.
NOTE
A ground flare is normally an enclosed flare, but may also refer to a ground multi-burner flare or a burn pit.
3.35
guyed flare
Elevated flare with the stack held in place using cables.
NOTE
A typical guyed flare is shown in Figure A.3.
6
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
3.36
heat release
Total heat liberated by combustion of the relief gases based on the lower heating value.
The heat release is expressed in kilowatts (British thermal units per hour).
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NOTE
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3.37
higher heating value
HHV
gross heating value
Total heat obtained from the combustion of a specified fuel at 16 °C (60 °F)
NOTE
The higher heating value includes the latent heat of vaporization of water in the combustion products (including the
water already present in the flare gas).
3.38
knockout drum
Vessel in the effluent handling system designed to remove and store condensed and entrained liquids from the
relief gases.
3.39
liquid seal
water seal
Device that directs the flow of relief gases through a liquid (normally water) on their path to the flare-burner, used
to protect the flare header from air infiltration or flashback, to divert flow or to create backpressure for the flare
header.
3.40
lower heating value
LHV
net heating value
Higher heating value minus the latent heat of vaporization of the water (both the water formed in the combustion
products and that already present in the flare gas).
3.41
Mach number
Ratio of a fluid’s velocity, measured relative to some obstacle or geometric figure, divided by the speed at which
sound waves propagate through the fluid.
3.42
management system
Framework of processes and procedures used to ensure that an organization can fulfill all tasks required to
achieve its goals and objectives.
NOTE
This is different from a combustion management system, which is typically an automated control system designed
to ensure safe combustion operation.
3.43
manifold
Piping system for the collection and/or distribution of a fluid to or from multiple flow paths.
3.44
multi-burner flare
Group of flare-burners designed to burn all or a portion of the design flow capacity, which are often arranged in
stages to facilitate better combustion.
NOTE
Multi-burner flares are capable of smokeless combustion at high flow rates with lower radiation levels.
3.45
muffler
Device that mitigates noise.
7
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
3.46
multi-point flare
Single flare-burner with multiple separate exits.
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3.47
non-reclosing pressure relief device
Pressure-relief device actuated by static differential or static inlet pressure.
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NOTE
Non-reclosing pressure device or any device approved by a recognized pressure relief authority, e.g. buckling pin,
rupture disk, etc.
3.48
opacity
Degree of non-transparency to rays of light.
NOTE
The opacity is typically quantified by the Ringelmann number.
3.49
pilot
pilot burner
Small, continuously operating burner that provides ignition energy to ignite and/or stabilize combustion of the
flared gases.
3.50
pressure design code
Recognized pressure equipment standard specified or agreed by the purchaser.
NOTE Examples include ASME VIII or EN 13445 (all parts) for pressure vessels and ASME B31.3 for piping or EN 13480
(all parts).
3.51
purge gas
Non-condensable flammable or inert gas added to the flare header to mitigate air ingress and burnback.
3.52
radiation intensity
Radiant heat flux from the flare flame.
NOTE
The heat flux rate is usually considered at grade level.
3.53
relief gas
waste gas
flared gas
waste vapor
Gas or vapor vented or relieved into a flare header for conveyance to a flare.
3.54
Ringelmann number
Visually comparative scale used to define levels of opacity, where clear is 0, black is 5 and 1 through 4 are
increasing levels of gray as used in describing smoke from combustion of hydrocarbons.
NOTE
The Ringelmann number is often used to describe the intensity of smoke.
3.55
riser
Pipe or other conduit that conveys the relief gas, combustion air, etc. to the flare-burner of an elevated flare.
3.56
smokeless capacity
Range of relief fluid flow to a flare-burner that can be burned without smoke.
8
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
NOTE
The term “without smoke” can be quantified using the Ringelmann number.
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3.57
staged flare
Group of two or more flares or burners that are controlled so that the number of flares or burners in operation is
proportional to the relief-gas flow.
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3.58
stoichiometric air
Chemically correct quantity of air, i.e. a quantity capable of perfect combustion with no unused fuel or air.
3.59
structural design code
Recognized structural standard specified or agreed by the purchaser.
NOTE
Examples include AISC S302, ASCE 7 and ASME STS-1.
3.60
supplemental gas
Combustible gas injected into the flare-gas header upstream of the flare-burner or burned external to a flareburner in order to facilitate the combustion of low-heating-value relief gas.
3.61
thermocouples
Temperature sensing devices used to detect whether the pilot is in operation.
3.62
velocity seal
orifice seal
Dry vapor seal that minimizes the required purge gas needed to protect against air infiltration into the flare-burner
exit.
3.63
wind fence
Structure surrounding a flare at ground level to modify the effect of crosswinds on the combustion process, to
prevent unauthorized access, limit the radiation to the surroundings and/or make the flame less visible.
See Figure A.8.
3.64
windshield
Device used to protect the outside of an elevated flare-burner from direct flame impingement.
NOTE
The windshield on a flare-burner is so named because external flame impingement occurs on the downwind side
of an elevated flare. Windshields are also integral to the design of pilots to avoid flame-outs during high winds.
4
4.1
Design
General
4.1.1
Being critical to the safety of an operating plant, a flare shall be continuously available and capable of its intended
performance through all operating-plant emergency conditions, including a site-wide general power failure as defined in the
flare datasheets.
4.1.2 The flare and related mechanical components shall be designed to operate and properly perform for the
specified service conditions for a minimum of five years without the need for an outage of the operating facility,
unless a shorter duration is specified by the purchaser.
NOTE
Flare availability and reliability are strongly dependent on the design of the related mechanical equipment and on
its installation, commissioning, maintenance and operation.
9
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
4.1.3
Management systems shall be in place that:
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a) clearly document the intended capacity, performance and operational limitations of the pressure-relieving and
vapor-depressurizing systems and flare;
b) provide operating procedures and operator training; and
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c) provide planned and routine maintenance of components critical to the safety and operating goals.
4.2
System Design
The following system design requirements shall be defined on the data sheets:
a) design-flow cases from the pressure-relieving and vapor-depressurizing system, including maximum
continuous case and maximum intermittent case as defined by mass flow and heat release;
b) flare staging requirement and method;
c) allowable flare-burner exit velocity;
d) system hydraulics with respect to allowable pressure drop, static pressure and diameter of the flare;
e) environmental performance requirements related to smokeless capacity, opacity limits, ground level
concentration of pollutants and permissible noise limits;
f)
operating performance such as peak radiation intensity at grade;
g) functional description of the intended system operation;
h) selection of major system components that can be integral to the flare such as a knockout drum, liquid seal,
purge-reduction device, etc.;
i)
site and ambient design conditions;
j)
utilities available; and
k) materials of construction.
NOTE 1
Also see 4.3 for process functional requirements.
NOTE 2 Fundamental system design requirements are established through consideration of the requirements and
guidance provided in API Standard 521.
4.3
Process Definition
4.3.1 In addition to the functional system design requirements as defined in 4.2, complete composition, range of
temperature and hydrocarbon characterization information of the process stream(s) shall be provided.
CAUTION In consideration of high-pressure flare systems, when the gas compositions and/or flow rates
of the relief gas are not considered compatible with the design requirements for high-pressure flareburners, the use of high-pressure flare-burner designs should be avoided. See A.2.6 for guidance on highpressure flare-burners.
4.3.2 In consideration that various operating or pressure-relieving cases can individually define various aspects
of the design; i.e. hydraulic capacity, ground-level radiation, aeration requirement for the defined smokeless
capacity, requirement for dilution gas, design metal temperature, thermal expansion, material selections, etc.;
multiple cases together with expected duration and frequency shall be provided to permit the designer to
determine which cases control the design.
10
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
4.3.3 The potential for liquid introduction or the condensation of hydrocarbons, or the formation of hydrates in
the flare header or flare stack, which can be carried to the combustion zone, shall be considered by both the user
and the flare designer.
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NOTE 1
Hydrocarbon droplets entrained in the gas stream that are carried into the flame usually burn incompletely, can
produce burning liquid droplets, form soot and decrease the smokeless capacity of the flare. The maximum liquid-droplet size
that can enter the combustion zone and can be handled within achievable measures for smokeless control depends on the
burner design. API Standard 521 gives further guidance on the maximum droplet size for different types of burners.
NOTE 2
4.4
Fundamental process definition is primarily established in API Standard 521.
Types of Flares
Through an understanding of the process, performance and operational requirements for the flare; and with
consideration of the mechanical, operability and maintenance implications for each; the designer shall specify the
most appropriate type of flare, configuration and components to meet the safety, operability and the functional
requirements established through use of this standard. Decisions on flare type and functional requirements shall
be documented in the data sheets.
NOTE 1
Refer to Figure A.1 for a general flare type selection guide. Within each general type of flare, various alternatives
and proprietary design aspects can exist. An understanding of alternatives and/or proprietary design aspects can be obtained
and evaluated using the data sheets in Annex F and instructions on their use in Annex E.
NOTE 2
For further guidance on the types of flares, the mechanical details and the requirements of design for each, refer to
Section 5 and Section 6 and Annex A, Annex B and Annex C. Decisions on functional requirements shall be documented in
the data sheets.
NOTE 3
4.5
For guidance on offshore flares see Annex D.
Flare-burners
A flare-burner supplied in accordance with this standard shall perform as specified under the defined service
conditions for a minimum of five years when installed and operated in accordance with the manufacturer’s
recommendations. Although many design alternatives for flare-burners exist, including those of a proprietary
nature, the following are the functional requirements that shall be met.
a) For all flare-burners:
 Flare-burners shall incorporate flame-retention devices or aerodynamic methods with proven capability to
provide stable combustion and protection from flame blow-off.
 Air or steam injection, if applied, shall not disrupt the basic flame stabilization mechanisms of the flareburner or pilot operation.
 Flare-burners in combination with their system of pilot(s)/ignition system(s) shall be capable of
maintaining stable combustion of the main flame for the specified service conditions, including the
environmental conditions as specified for pilots in this standard.
b) For flare-burners with internal steam injection to induce air:
 Steam condensate shall be drained from the internal steam/air injection point and from any muffler
surrounding these tube assemblies.
c) For staged flare-burners:
 If flare-system safety considerations includes valve bypasses, then each of which shall contain a fail-safe
device if the flare is the final relief protection for the system, e.g. rupture disc, non-reclosing pressure
relief device, or any device that is approved by a recognized pressure relief authority.
11
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
4.6
Mechanical Design
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4.6.1 There are numerous physical arrangements and mechanical designs available from which to select the
most appropriate for the application, in accordance with the local codes and regulations. Whichever selection is
made, the following shall apply.

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a) The pressure design code shall be specified or agreed by the purchaser. Pressure components shall comply
with the pressure design code and the supplemental requirements in this standard.

b) The structural and mechanical design codes shall be specified or agreed by the purchaser. Structural
components shall comply with the structural design code and the supplemental requirements in this standard.

c) The purchaser and the vendor shall mutually determine the measures required to comply with any local or
national regulations applicable to the equipment.

d) The purchaser shall specify the design temperature and design pressure.
NOTE 1
For uninsulated flare stacks, it is not necessary to consider that the maximum design temperature and wind loads
apply simultaneously because of the cooling effect of the wind. For the design wind case, a credible metal temperature under
design wind conditions shall be established, listed on the data sheet and used in the design. For vertical thermal expansion
analysis, the maximum operating temperature shall be used as the design basis.
NOTE 2
For insulated or shielded flare stacks, design temperature and wind loads shall be considered to apply
simultaneously.
NOTE 3
Further guidance on metal temperature considerations is given in E.3.3, Form Elev 4.
4.6.2 The following shall be taken into account in the mechanical design, fabrication, inspection and testing of
the mechanical components of a flare:
a) flared stream temperature, pressure and composition;
b) corrosive nature and liquid content of the flared stream;
c) ambient conditions;
d) site conditions;
e) wind, snow and ice loading;
f)
seismic loading;
g) jet loads associated with high-velocity flare-burners;
h) cyclic loading, vibration (fatigue);
i)
hydraulic forces, as are possible with blow-dry liquid seals;
j)
pressure waves, as are possible with staged flares;
k) proximity to equipment, structures, thoroughfares and site boundaries;
l)
maintenance and inspection criteria;
m) contraction/expansion of stack due to ambient or process changes;
n) pressure due to flashback, if specified by the purchaser;
NOTE
The use of a properly designed, reliable purge can preclude the requirement to design for flashback pressure.
12
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
o) bolting.
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NOTE
Measures should be taken to prevent bolting from coming loose due to vibrations, etc. The use of bolts and double
nuts is recommended, whereas the use of welded‑ on nuts is not. Alternatives to double nuts, if specified, include locking or
cupped spring-type washers. Where size does not allow the use of bolts, use stud bolts with double nuts on both sides. This
applies to any bolted connection, regardless of its size, location or use.
4.7
Pilots
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4.7.1 There are numerous pilot designs available from which to select the most appropriate for the application.
Although design alternatives exist, the following are the functional requirements that shall be met.
a) Pilots shall be continuously burning.
b) Pilots shall reliably ignite the flare flame on single- and multi-burner flares.
c) Minimum pilot heat release shall be 13.2 kW (45,000 Btu/h).
d) Pilots shall remain burning, even if the flaring gases are not flammable.
e) Pilots shall remain burning and capable of being re-ignited at wind speeds up to 160 km/h (100 mph) under
dry conditions and 140 km/h (85 mph) when combined with at least 50 mm (2 in.) of rainfall per hour. This
performance shall be verified by type testing in accordance with a documented test protocol and documented
results.
NOTE
f)
The recommended functional elements for verification testing are provided in A.6.
(●) The minimum number of pilots shall be in accordance with Table 1.
Table 1 — Number of Pilots for Single-point Flares
Minimum Number
of Pilots
Flare-burner
Outlet Diameter
(DN)
Flare-burner
Outlet Diameter
(NPS)
1a
up to 200
up to 8
2
 200, up to 600
 8, up to 24
3
 600, up to 1050
 24, up to 42
4
 1050, up to 1500
 42, up to 60
b
 1500
NOTE 1
greater.
 60
3
For flares with relief-gases of 12 MJ/Nm (300 Btu/scf) or
NOTE 2 For low heating value and/or toxic services, additional
pilots may be required.
a For toxic gas, the minimum number shall be two.
b To be agreed with the purchaser.
g) Pilot tip and components exposed to flame shall be constructed of a heat-resistant material, e.g. AISI 309,
310, 310H, etc.
h) For self-aspirating pilots, the air inlet shall be located so that it has uninterrupted air access and shall be at
least 1.8 m (6 ft) or 125 % of the actual burner diameter (whichever is greater) from the top of the flare.
i)
For self-aspirating pilots, a strainer shall be installed at grade for protection of the pilot regulator. A strainer or
settling chamber shall also be installed just upstream of the gas orifice, unless the piping between the grade
strainer and the gas orifice is stainless steel.
13
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
j)
Piping and components between the pilot tip and the air mixer shall be constructed of austenitic stainless
steel.
FO
k) Pilots shall be designed for the specified fuel gas supply.
l)
A continuous source of clean fuel, which is regulated in pressure and has a defined range of heating value
and composition (e.g. natural-gas quality), shall be supplied to the pilot.
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m) Each pilot shall have at least one dedicated means of ignition and one dedicated means of pilot-flame
detection.
n) If the pilot venturi has an adjustable air door, ensure that there is a reliable locking mechanism for the air door.
o) The pilot ignition and detection systems shall be selected with consideration of the intended maintenance
intervals of the pilots.
NOTE
For guidance on pilot selection, maintenance and troubleshooting, refer to A.3.
p) Individual fuel-supply lines shall be installed to each pilot, if required, to improve operability and reliability of
the pilots.
4.8
Pilot-ignition Systems
There are numerous pilot ignition system designs available from which to select the most appropriate for the
application. Although design alternatives exist, the following are the functional requirements that shall be met.
a) The pilot ignition system shall be able to reliably ignite the pilot, including at wind speeds of up to 160 km/h
(100 mph) under dry conditions and 140 km/h (85 mph) when combined with at least 50 mm/h (2 in./h) of
rainfall. This performance shall be verifiable by type testing in accordance with a documented test protocol
and documented results.
NOTE
The recommended functional elements for verification testing are provided in A.6.
b) The pilot ignition system shall be able to ignite the pilot during all defined operating and emergency relief
cases, including a site-wide general power failure.
c) It shall be possible to ignite each pilot independently of the other pilots without depending on the flare flame
for ignition.
d) Compressed-air flame-front generator systems shall be designed to prevent back-flow of one utility system
into another.
e) FFG systems shall be designed to ensure that the ignition lines after pilot ignition shall be free from ignitable
mixture that may cause a stable flame inside the lines.
NOTE
4.9
For guidance on pilot ignition equipment selection, maintenance and troubleshooting, refer to A.4.
Pilot-flame Detection
There are numerous pilot-flame detection system designs available from which to select the most appropriate for
the application. Although design alternatives exist, the following are the functional requirements which shall be
met.
a) The pilot-flame detection system shall be able to distinguish between pilot flame and flare-burner flame.
b) The pilot-flame detection system shall be able to detect the pilot flame, including at wind speeds of up to
160 km/h (100 mph) under dry conditions and 140 km/h (85 mph) when combined with at least 50 mm/h
14
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
(2 in./h) of rainfall. This performance shall be verifiable by testing in accordance with a documented test
protocol and documented results. A typical test protocol is given in A.6.
c) Each pilot shall have at least one dedicated means of pilot-flame detection.
FO
NOTE
For guidance on pilot-flame detection equipment selection, maintenance and troubleshooting, refer to A.5.
4.10 Piping
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4.10.1 Design criteria for auxiliary piping attached to the flare stack or support structure shall be in accordance
with the pressure design code or the superior requirements of a user’s or flare manufacturer’s specification.
See 4.6.
4.10.2 The material of the flare piping system shall be selected on the basis of relief gas compositions,
temperature, pressure and any potential impact of ambient conditions.
4.10.3 Flanges shall be kept to a minimum since they can be a source of leakage.
4.10.4 Piping shall be evaluated for differential thermal expansion due to the temperature gradient between the
piping and the support structure/flare stack. If necessary, expansion loops shall be used in the piping on the stack
and, in particular, on steam lines.
4.10.5 Expansion joints shall be avoided due to their tendency to leak and the difficulties in their maintenance.
4.10.6 Expansion joints shall not be used in lines containing flammable and/or toxic gases.
NOTE
Any required movement of the auxiliary piping interface point should be specified by the flare system designer and
accommodated in the inlet piping by the purchaser.
4.10.7 All piping shall be considered in the structural analysis of the wind and earthquake loads, including any
insulation (whether applied before shipment or applied on site).
4.10.8 Piping shall be designed for all operating and test loads, including water-filled steam piping.
4.11 Auxiliary Components
4.11.1 There are numerous auxiliary components available from which to select those that are appropriate for
the mechanical design considerations of the flare. The following are the functional requirements that shall be met.
a) The flare stack structural design shall consider the loads due to any auxiliary component directly attached to
or incorporated into the design flare stack, e.g. ladders and platforms, axial blower and ducting, buoyancy
seal, seal drum, etc.
b) For parallel flares, at least one liquid seal or some other means of isolation shall be provided downstream of
the branch between the two flares.
c)
Flares in cold service or sub-freezing due to process concerns shall be equipped with means to prevent
blockage that can be caused by the freezing of trapped water. Special attention shall be given to liquid seals
and buoyancy seals.
d) Where there is the potential for solids formation, hydrates or liquid freezing under cold-climate conditions,
heat-tracing system requirements shall be considered.
e) For an air-assisted flare in which multiple blowers are used, the full operating sequence, blower staging and
system hydraulics shall be engineered in detail from start-up through their maximum flow operation.
f)
For an air-assisted flare in which multiple blowers are used, the air-side system shall be designed with
isolation dampers for any blower that is out of service.
15
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
4.11.2 Whenever buoyancy or diffusion-type purge seals are used:
FO
a) They shall be equipped with a drain that is adequately sized to prevent plugging and be sealed to prevent the
entry of air.
b) They shall be equipped with an inspection opening to allow cleaning of the drain during shutdowns.
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c) The drain line shall be protected against freezing.
NOTE
seals.
See A.7 and API Standard 521G.6.1 for further guidance and design requirements for purge-gas conservation
4.11.3 When electric motor drives are used for blowers on air-assisted flares, the thermal overload settings and
breaker settings shall take into account winter operation.
5
Mechanical Details — Elevated Flares
5.1
Mechanical Design — Design Loads
The design of the support structure for both single-burner and multi-burner vertical elevated flares shall consider,
as a minimum, the design loads defined below. The supplier shall advise and confirm with the purchaser before
award of any exceptions that they may have or consider as not applicable to the specific application.
NOTE
A general description of several methods that can be used to support a vertical flare are provided in A.1.
a) Wind loadings:
Wind loading shall take into account the flare stack and all of its appurtenances such as, but not limited to,
piping (including insulation, if any), access platforms and ladders. When appropriate, the wind loading on
supporting derrick structures shall be included. Wind loads shall be based on the local regulations, e.g.
ASCE 7.
b) Earthquake-induced loads:
Earthquake-induced loads shall be based on the local regulations, e.g. ICBO or ASCE 7. Structural design
shall be based on wind loads and earthquake loads occurring separately.
c) Wind-induced vibration loads:
The structural design for wind-induced vibration shall be in accordance with the ISO limit-state method in
API Standard 560, Annex H or ASME STS-1.
d) Internal pressure:
1) The purchaser shall specify the internal pressure for the flare stack.
NOTE 1 Flare stacks are generally operated at near-atmospheric pressure and are not specifically designed as pressure
equipment. Exceptions to this include conditions when the flare-burner creates a backpressure in the flare stack that
causes the pressure design code to become applicable.
NOTE 2
In some cases, the bottom portion of the stack incorporates a knockout drum or liquid seal, or both.
2) When specified in the data sheets, the stack portion containing the knockout drum or liquid seal shall be
designed as a pressure vessel including, if specified, hydrostatic testing and code stamping. In such
cases, the design of the pressure vessel portion shall be in accordance with the pressure design code
and shall be based on the design pressure stated in the data sheets or at least 105 kPa (ga) (15 psig).
16
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
NOTE
It is common practice that the stack be designed for structural loads and the anticipated maximum operating
pressure when the base section of the stack is designed as a pressure vessel. For design purposes, the stack is deemed
to start at the pressure vessel outlet.
FO
3) The design of a stack base section that is a pressure vessel shall include provision for internal pressure
loads and for simultaneous wind loads based on 70 % of the design wind velocity.
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4) The design pressure (maximum allowable working pressure) of a stack-base section that is a pressure
vessel shall not be increased due to any material thickness that is in excess of that thickness required to
meet the design pressure.
e) Jet loads:
1) In consideration of sonic-type flares with an exit velocity greater than Mach 0.8, special considerations
related to vibration and fatigue shall be considered in the mechanical design. Refer to A.2.6 for guidance.
2) (●) The requirement for vibration and fatigue analysis shall be mutually agreed between purchaser and
supplier.
f)
(●) Flashback pressure:
If the purchaser has specified that flashback pressure be taken into account, the pressure used in the
calculations shall be specified by the purchaser.
g) (●) Nozzle loads:
1) The purchaser shall specify the preliminary flare-header-imposed nozzle loads and elevation on the data
sheets furnished to the vendor with the inquiry.
2) The purchaser shall provide final flare-header-imposed nozzle loads and elevation during the design
phase. If the purchaser does not furnish nozzle-load information, the flare supplier shall use the nozzleallowable forces, moments and elevation specified in Table 2.
3) The purchaser shall also define the loads and moments of the external piping nozzles (for steam, fuelgas-to-pilot, etc.). If the purchaser does not impose them, then the designer shall select them.
NOTE 1 Nozzle loads are imposed on the stack by the flare-header-to-stack connection and can have a major influence
on the design of the flare stack. This influence increases as the elevation of the flare-header increases. In general, the
flare stack itself should not be used as a fixed anchor point for the flare-header piping.
NOTE 2 The purchaser should also minimize the nozzle loads imposed on the stack by other service connections.
h)
Thermal loads:
1) In consideration that the relief-gas cases described in the data sheets can have a gas temperature that is
different from ambient, the flare stack designer shall provide for thermal expansion and contraction that
can be caused by a specified gas temperature or by temperature differences caused by wind or rain.
NOTE
Thermal loads are of greatest concern on guyed stacks and derricks.
2) Special attention shall be given to the design of stacks where the stack wall temperature is expected to be
175 °C (350 °F) or higher.
NOTE 1 Temperature differential between a hot stack wall and varying ambient conditions, for instance, by wind rain on
one side of the stack, can cause the stack to buckle or bend, inducing additional stress on the stack, guy wires or derrick
and foundations. Counter-measures include shielding or insulating of the stack.
NOTE 2 The freedom of movement of a self-supported stack reduces the thermal load considerations, providing that the
thermally induced deflection does not exceed 450 mm per 30 m (18 in. per 100 ft).
17
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
3) The designer shall provide for thermally induced differential changes in length between the stack and the
piping attached to the stack.
FO
i)
Erection and/or maintenance loads:
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1) The flare stack designer shall be informed of the erection techniques that the purchaser plan to employ. If
the stack is being erected in a single lift, the designer shall also be informed as to the degree of preerection attachment of piping, ladders and platforms, etc.
2) Unless otherwise noted in the data sheets, lifting trunnions shall be designed for one-time use.
3) Wherever derrick-supported flares are designed to allow the flare riser to be lowered (either as a single
section or in multiple sections), the designer shall provide for the loads imposed on the flare riser and
derrick by the intended plan of demounting.
4) Wherever a davit, located near the top of the flare stack, is used to lower the flare-burner to grade, the
designer shall determine and provide for the davit-imposed loads based on the intended plan for this
operation.

5) When specified by the purchaser, temporary platforms with relevant support for erection activities at
elevation shall be considered in the structural design. Lugs, brackets or reinforcements for erection
platforms shall be properly located and designed according to the erection method and sequence.
j)
Auxiliary loads:
Auxiliary loads to the flare stack shall be incorporated into the structural design, e.g. those associated with
attachment of blowers, drivers and ducting as appropriate for air-assisted flares.
k) Special loads:
The purchaser shall record on the data sheets any additional loads that will be imposed on the flare
stack/support structure.
NOTE
Examples of special loads include:
i)
loads from ice buildup;
ii)
hydrostatic testing of ancillary components;
iii)
loads imposed during transport;
iv)
dead loads including liquid in KO drum, water seal, etc.;
v)
hydraulic forces that are possible with blow-dry liquid seals and
vi)
pressure waves that are possible with staged flares.
NOTE
Hydraulic shock load is not generally considered for design.
18
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
Table 2 — Allowable Forces and Moments for Flare Nozzles
Nozzle size
DN (NPS)
Fx
Fy
Fz
Mx
My
Mz
(lbf)
N
(lbf)
N
(lbf)
Nm
(ftlbf)
Nm
(ftlbf)
Nm
(ftlbf)
50 (2)
445
100
890
200
890
200
475
350
339
250
339
250
100 (4)
890
200
1779
400
1779
400
813
600
610
450
610
450
125 (5)
1001
225
2002
450
2002
450
895
660
678
500
678
500
150 (6)
1112
250
2224
500
2224
500
990
730
746
550
746
550
200 (8)
1334
300
2669
600
2669
600
1166
860
881
650
881
650
250 (10)
1557
350
2891
650
2891
650
1261
930
949
700
949
700
300 (12)
1779
400
3114
700
3114
700
1356
1000
1017
750
1017
750
350 (14)
2003
450
3338
750
3338
750
1451
1070
1085
800
1085
800
400 (16)
2225
500
3560
800
3560
800
1546
1140
1153
850
1153
850
450 (18)
2448
550
3783
850
3783
850
1641
1210
1221
900
1221
900
500 (20)
2670
600
4005
900
4005
900
1736
1280
1289
950
1289
950
600 (24)
3115
700
4450
1000
4450
1000
1926
1420
1424
1050
1424
1050
700 (28)
3560
800
4895
1100
4895
1100
2116
1560
1560
1150
1560
1150
750 (30)
3783
850
5118
1150
5118
1150
2211
1630
1628
1200
1628
1200
800 (32)
4005
900
5340
1200
5340
1200
2306
1700
1695
1250
1695
1250
900 (36)
4450
1000
5785
1300
5785
1300
2496
1840
1831
1350
1831
1350
1000 (40)
4895
1100
6230
1400
6230
1400
2686
1980
1967
1450
1967
1450
1100 (44)
5340
1200
6675
1500
6675
1500
2875
2120
2102
1550
2120
1550
1200 (48)
5785
1300
7120
1600
7120
1600
3065
2260
2238
1650
2238
1650
1300 (52)
6230
1400
7565
1700
7565
1700
3255
2400
2374
1750
2374
1750
1400 (56)
6657
1500
8010
1800
8010
1800
3445
2540
2509
1850
2509
1850
1500 (60)
7120
1600
8455
1900
8455
1900
3635
2680
2645
1950
2645
1950
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N
Key
1
a
b
Nozzle
Nozzle loads are applicable for the flare header to the gas riser/stack, but not applicable for auxiliary service piping, e.g. steam, pilot
fuel, etc. The location of the relief-gas nozzle as a basis for design is 10 m (32.8 ft) above grade.
Values for nozzle sizes 50 mm (2 in.) to 300 mm (12 in.) are taken from API Standard 560, 4th Ed., 2007, Table 7. For nozzles
greater than 300 mm (12 in.), the values are extrapolated from this same source.
19
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
5.2
Design Details
FO
The following design details shall be incorporated into the support structure design.

a) Unless otherwise specified, stack deflection due to wind load or earthquake shall be:
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 structures: 300 mm per 30 m (12 in. per 100 ft),
 derrick supported structures: 150 mm per 30 m (6 in. per 100 ft),
 self-supported structures: 225 mm per 30 m (9 in. per 100 ft).
b) Guyed Stack sections in the form of a frustum of a cone shall be designed in accordance with
API RP 2A WSD.
c) Field assembly of the stack sections shall be by welding, unless otherwise specified on the flare data sheets.
Provisions for erection activities at elevations (e.g. lugs, brackets or reinforcement for temporary platforms)
shall be welded.
NOTE
A flanged assembly may be used on multiple-section, demountable riser sections of the stack.
d) Corrosion allowances:
1
1) A minimum corrosion allowance of 1.6 mm ( /16 in.) for carbon steel components in contact with the relief
gas stream shall be added to the minimum metal thickness necessary to meet the requirements of 5.1.
2) No corrosion allowance is required on alloy steel portions of the stack, unless specifically noted on the
data sheets.
3) Non-alloy steel internal parts of the liquid seal or knockout drums that are normally in contact with the seal
fluid or relief gas stream shall have the corrosion allowance applied to each side.
4) No corrosion allowance is required for derrick structures and other metal objects that do not normally
come into contact with the relief gas stream.
e) The following requirements shall be incorporated when the support structure design includes the use of guy
wires:
1) Guy-wire slope and initial tension shall take into account:
 the elevation of the guy connections to the stack (there may be more than one connection elevation);
 available guy-wire anchor radius;
 the range of stack temperatures, both ambient and process;
 wind loads;
 mass distribution.
2) There are two types of cables used for guy wires: wire rope and structural bridge strand, with the following
acceptable supply specifications:
i)
wire rope, in accordance with:
I)
3
ISO 2408:2004, Quality B for 10 mm to 50 mm ( /8 in. to 2 in.) diameter, class 6X36-IWRC with
steel core;
20
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
5
II) ASTM A475-03, Class A for 16 mm ( /8 in.) diameter or less, with metallic core.
ii)
structural bridge strand, in accordance with:
FO
I)
1
EN 12385-10 for 13 mm to 72 mm ( /2 in. to 3 in.) diameter, with zinc coating conforming to
EN 10264-2:2002, Class A;
5
II) ASTM A586-04a, Class A, for diameters greater than or equal to 16 mm ( /8 in.).
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NOTE
Refer to ASME STS-1 for cable selection criteria.
3) End fittings, turnbuckles, rods and pins shall have a load capacity suitable for the maximum guy wire
tension expected. Guy wire hardware shall be strong enough to withstand a force at least equal to the
minimum breaking force of the guy wire.
4) The initial guy wire tension shall not be less than 2 % or greater than 12.5 % of the maximum guy wire
tension expected. Initial guy wire tension shall include a consideration of the ambient temperature
variation and loads offset along the flare stack.
5) Turnbuckles or guy wire tension devices shall be located at the anchor end of the guy wire immediately
above the anchorage. Locking devices shall be included with all turnbuckles and tension devices.
6) When guy wire deadmen (anchors) cannot be located symmetrically around the stack and at the same
elevation, the analysis shall determine the load direction that provides the highest stress for all
components, in addition to the design load analysis requirements in 5.1.
5.3

Flanges
5.3.1 When flanged flare-burner connections are specified by the purchaser, flanges shall be as follows:
a) for DN 600 (NPS 24)
PN 20 Type 01;
and
smaller
burners:
ASME B16.5,
Class 150 RFSO,
or
EN 1092-1:2007,
b) for sizes greater than DN 600 (NPS 24): forged flanges or fabricated plate flanges drilled to industry standard
dimensions as specified in Table 3; for flange sizes greater than those shown in Table 3, follow the flare
manufacturer’s standard;
c) auxiliary connections such as those less than DN 600 (NPS 24) for steam and natural gas: flange ratings in
accordance with ASME B16.5.
NOTE 1
See A.2.7.1 and A.2.7.2 for further guidance on flare-burner and auxiliary connections.
NOTE 2
See 5.4 for flange metallurgical requirements.
21
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
Table 3 - Flange-bolting Dimensions for Flare-burners Larger than DN 600 (NPS 24)
Bolt Circle
Diameter
mm (in.)
No. of
Bolt Holes
Bolt Hole
Diameter
mm (in.)
650 (26)
758.8 (29.875)
28
22.2 (0.875)
700 (28)
809.6 (31.875)
28
22.2 (0.875)
750 (30)
866.8 (34.125)
36
22.2 (0.875)
800 (32)
917.6 (36.125)
36
22.2 (0.875)
850 (34)
974.7 (38.375)
36
25.4 (1.000)
900 (36)
1025.5 (40.375)
36
25.4 (1.000)
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Flare-burner Size
at Connection a
DN (NPS)
950 (38)
1076.3 (42.375)
36
25.4 (1.000)
1000 (40)
1127.1 (44.375)
40
25.4 (1.000)
1050 (42)
1190.6 (46.875)
40
28.6 (1.125)
1100 (44)
1241.4 (48.875)
40
28.6 (1.125)
1150 (46)
1292.2 (50.875)
40
28.6 (1.125)
1200 (48)
1343.0 (52.875)
44
28.6 (1.125)
1250 (50)
1393.8 (54.875)
44
28.6 (1.125)
1300 (52)
1454.2 (57.250)
44
31.8 (1.250)
1350 (54)
1505.0 (59.250)
44
31.8 (1.250)
1500 (60)
1657.4 (65.250)
48
31.8 (1.250)
NOTE
If fabricated plate flanges are utilized, the mechanical design calculations
should be made available if required by the end user.
a

5.3.2 Air ducting associated with air-assisted flares shall be assembled using the manufacturer’s standard plate
flanges including air connections at the flare-burner.
NOTE

Bolting dimensions are based on “Industry Standard” flanges, Class 175.
The mechanical design calculations for these flanges should be made available if required by the end user.
5.3.3 When specified by the purchaser, measures shall be taken to prevent bolting from coming loose due to
vibrations, etc. For further guidance, see A.2.7.2.
NOTE 1
For sonic tips (Mach >0.8), such measures are required.
NOTE 2
The use of bolts and double nuts is recommended, whereas the use of welded-on nuts is not. Alternatives to
double nuts, if specified, include locking or cupped spring-type washers. Where size does not allow the use of bolts, use stud
bolts with double nuts on both sides.
5.4
Materials of Construction
5.4.1 Materials of construction shall be as specified in the flare data sheets. If a material is not specified, any
portion of the flare stack that is normally exposed to the relief gases shall be of a material that is compatible with
the gas composition as stated in the flare data sheets. In addition, the materials normally exposed to the relief
gases shall be compatible with the most extreme maximum and minimum temperatures imposed by the relief
gases, the ambient temperatures or the design temperature(s) specified in the flare data sheets.
NOTE
Portions of the flare stack or the support structure that are not normally exposed to the relief gases may be of a
material suitable for their purpose.
5.4.2
NOTE
The flare-burner inlet flange shall meet the metallurgical requirements of the flare stack.
As an example, with a carbon steel stack, a carbon steel burner flange is acceptable.
22
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
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the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
5.4.3 Welded attachments to the portions of the stack that carry the relief gases shall be of a material that is
compatible with the material of the respective gas-containing portion.

FO
5.4.4 The purchaser shall specify whether the service is sour (i.e. whether sulfide stress cracking is possible) in
accordance with NACE MR0103 for other applications (e.g. oil refineries, LNG plants and chemical plants), or in
accordance with NACE MR0175 (all parts) for oil and gas production facilities and natural gas industries, in which
case all materials in contact with the process fluid shall meet the requirements of that standard.
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5.4.5 Derrick structures shall be designed for fabrication from members that are structural shapes, pipe or plate
or any combination thereof.
5.4.6 Guy wires, as set forth in 5.2 (e), shall be all metallic, galvanized steel wire rope or structural bridge
strand. Guy wire hardware shall be galvanized. Galvanizing shall be at least equivalent to the guy wires on which
they are used.
5.4.7 Materials used for portions of the support structure that are designated as being designed according to
the pressure design code shall be in accordance with material specification requirements of that code.
5.5

Welding
5.5.1 Relief-gas-containing portions of the support structure shall be fabricated in accordance with the welding
requirements of the structural design code or pressure design code.
5.5.2 If the bottom portion of the stack is designed in accordance with the pressure design code, then the
fabrication of that portion shall be governed by the welding qualification requirements of the pressure design code.

5.5.3 Non-gas-containing portions of the support structure shall be fabricated in accordance with the welding
requirements of either the structural design code or the pressure design code.
5.6
Inspection
5.6.1 Relief-gas-containing portions of the support structure shall be inspected in accordance with the following
requirements.

a) The structural design code or pressure design code shall be used for non-destructive testing and inspection
procedures, techniques, standards for acceptance, inspector qualification and inspections.
b) All welds shall be visually inspected during the welding operation and again after the work is completed to
determine satisfactory penetration of weld metal fusion and satisfactory operator performance. After the
welding is complete, welds shall be cleaned before the inspector performs the final weld inspection. The
inspector shall pay particular attention to surface cracking, surface porosity, surface slag inclusion, undercut,
overlap, gas pockets and size of weld. Defective welding shall be corrected according to the applicable code
requirements.
c) Radiographic weld inspection shall be performed on all full-penetration structural butt welds to the extent
specified in the following requirement.
1) A minimum of one radiograph per each three shop circumferential seams on the relief-gas-containing
structural shell shall be carried out at the vertical weld intersection.
NOTE
The developed radiographic film should show at least 150 mm (6 in.) of weld seam.
d) Other types of non-destructive testing may be utilized if specified on the flare data sheets.
e) Hydrostatic testing is not required, unless specified on the flare data sheets.
f)
If the bottom portion of the stack is to be designed and fabricated in accordance with the pressure design
code, then the bottom portion shall be inspected in accordance with that code. In the case where the bottom
23
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
portion is a code vessel, the upper (non-code) portion shall be inspected as set forth in (a) through (e), above.
Hydrostatic testing is not required, unless specified by the flare data sheets or a code stamp is specified.
FO
5.6.2 Non-gas-containing portions of the support structure shall be inspected visually using the procedure set
forth under b), above.
Surface Preparation and Protection
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5.7
5.7.1 Carbon steel external surfaces of stack and piping that can be directly exposed to weather shall be
cleaned in accordance with SSPC SP 6/NACE No. 3. The cleaned surfaces shall then be primed with one coat of
inorganic zinc primer to a minimum dry film thickness (DFT) of 75 µm (0.003 in.). Surfaces shall be painted in
conditions in accordance with manufacturer’s recommendations on temperature and relative humidity.
5.7.2 Components of derricks, ladders, platforms, etc. shall be prepared and galvanized in accordance with the
applicable sections of ASTM A123/123M, ASTM A143/143M, ASTM A153/153M, ASTM A384/384M and
ASTM A385/385M. Bolts joining galvanized sections shall be galvanized in accordance with or ASTM A153/153M
or zinc-coated in accordance with ASTM B633.

5.7.3 Unless otherwise specified, alloy steel components do not require surface preparation or protection.

5.7.4 When specified by the purchaser, ladders and platforms may be painted as an alternative to galvanizing.
5.8
Attachments
5.8.1
Attachments to the support structure shall not be supported by the flare-burner.
NOTE
Typical attachments to the support structure include piping for steam, pilot gas and assist gas, buoyancy seal drain,
muffler drains, pilot ignition, electrical conduits and instrumentation conduits.
5.8.2 The flare structural analysis shall include piping loads and wind and earthquake loads in consideration of
the attachments.
5.9

Aircraft Warning Lighting
5.9.1 Marking on the support structure shall be in accordance with the code specified by the local aviation
authority.
NOTE
Markings are typically the national incorporation of ICAO, Annex 14.
5.9.2 Warning lights on the structure shall be fixed or retractable. Warning lights and electrical cables shall be
shielded from radiation as necessary and designed for appropriate temperature.
NOTE
required.
Depending on the type of lighting used and the height of the structure, special painting of the structure may be
5.10 Platforms and Ladders

5.10.1 The purchaser shall specify whether platforms are required. A ladder is the preferred means of access
to a platform, but alternative access can be achieved via a crane basket or helicopter. The purchaser shall specify
whether ladders are required.
NOTE
For guidance on the provision of ladders and platforms, refer to A.1.9.
5.10.2 Ladders or platforms shall not be attached to the flare-burner.
5.10.3 Platform decking shall be open grating, designed for a uniformly distributed load of 500 kg/m2 (100 lb/ft2)
and a point load of 450 kg (1000 lb).
24
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
5.10.4 Platforms and ladders shall be hot-dip galvanized carbon steel or shall be stainless steel.
5.10.5 Design of the ladders and platforms shall comply with local regulations.
FO
5.10.6 Ladder safety gates in areas exposed to high flame radiation shall be galvanized or painted steel.
6
Combustion Chamber
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6.1
Mechanical Details — Enclosed-flame Flares
6.1.1 The combustion chamber design shall conform to an acceptable code, e.g. ASCE 7 or EN 1993 using the
site parameters for wind speed, rain, exposure factor, seismic factor, etc.
NOTE 1
Mechanically, the combustion chamber is a self-supported stack. Ladders and service platforms, for access to
enclosed-flame flare instruments and stack emission sampling, can impact the structural design of the combustion chamber.
NOTE 2
6.1.2
For further guidance on the mechanical considerations for enclosed-flame flares, refer to C.2.3.
The stack design and material shall accommodate the thermal requirements of the enclosed flame.
6.1.3 If the exterior frame of the combustion chamber is painted and exceeds 205 °C (400 °F) during design
operation, an internal refractory lining shall be provided to limit the temperature of the external shell. Where
refractory lining is applicable, the refractory system design and material selections shall consider:
a) peak operating temperature (with a safety factor);
b) exterior shell temperature limits for materials and coating;
c) thermal cycling with rapid increase and decrease of combustion chamber operating temperature with changes
in relief-gas flows to the flare;
d) velocity of the airflow into the combustion chamber and the flue-gas velocity out of the combustion chamber;
e) environmental exposure to rain, wind, etc.;
f)
refractory weight, friability, expansion/contraction factors, durability, maintainability and service life;
g) refractory curing schedules and start-up plans;
h) compatibility with waste-gas composition;
i)
the need for an internal protective coating under the refractory lining.
NOTE
Internally, the combustion chamber temperature is established by the flow of relief gases through the burners and
how combustion and quench air are supplied to the flames. Most enclosed-flame flares are designed to operate with a
maximum internal temperature of about 980 °C to 1090 °C (1800 °F to 2000 °F).
6.1.4 Personnel protection shall be provided adjacent to the combustion chamber when surface temperatures
exceed 80 °C (175 °F) for areas for personnel access during operation.
6.2
Burners
6.2.1 The design of burners for enclosed-flame flares is considered proprietary to the manufacturer. However,
inherent to all enclosed-flame flares, the burner design shall:
a) be engineered for the relief-gas flow rates and compositions and possible pressure and temperature ranges of
the gases;
b) consider the utilities that are available for burner operation;
25
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
c) consider known likely operating conditions;
d) operate at all airflow rates and velocities within the design range;
FO
e) achieve the desired level of combustion emissions with flame volumes that are contained within the
combustion chamber;
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f)
have a stable flame within the range of operating airflow rates and velocities in the combustion chamber
design;
g) consider cross-lighting capability when there is more than one burner, unless each burner has an individual
pilot;
h) include the required airflow rate within the design for the combustion chamber;
i)
produce a stable flame for all relief-gas flow conditions and compositions within the design parameters;
j)
not induce any excessive combustion rumble that can trigger excessive noise and resonance from the
combustion chamber.
NOTE
Experience with a burner design operating as part of a complete design of an enclosed-flame flare is
recommended.
6.2.2 The selection of the flare-burner assembly connection shall take into account the composition and
temperature of the relief gas and possible exposure to high thermal loads from proximity to the flames or the
combustion chamber. Connection types include screwed fittings, flanged fittings, or welded.
NOTE
Temperature and vibration effects can loosen some screwed and flanged connections.
6.2.3 Steam, air or other auxiliary connections to the burner shall take into consideration their location relative
to the flame and combustion chamber.
6.3
Burner Piping
6.3.1
Burner piping shall conform to the pressure design code (e.g. ASME B31.3, as a minimum).
6.3.2
The piping shall be engineered to accommodate thermal expansion of the combustion chamber.
6.3.3
Piping material selection shall be compatible with the relief-gas composition and temperature.
6.3.4 The piping design shall maintain an acceptable burner position with respect to the air inlet to the
combustion chamber.
6.4
Pilots
6.4.1 Each stage shall be equipped with at least one pilot.

NOTE 1
The design may have additional pilots based on process demands and flare design.
NOTE 2
For stage 1, this may include one pilot per burner.
6.4.2 The number of pilots per stage shall be defined by the vendor and approved by the purchaser.
NOTE 1
Depending on the burner spacing and design, a single pilot may be used to light one main burner and cross-ignite
other burners.
NOTE 2
Many of the pilot and pilot-ignition details covered in A.3 and A.4 are applicable to enclosed-flame flares.
NOTE 3
Refer to C.2.6 for further guidance on design and specification of pilots for enclosed-flame flares.
26
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
6.5
Wind Fence
6.5.1 The wind fence structural design shall conform to an acceptable code (e.g. ASCE 7 or EN 1993) using the
site parameters for wind speed, rain, exposure factor, seismic factor, etc.
FO
6.5.2 The wind-fence design shall withstand heat, mitigate wind effects, limit access and provide acoustical
dampening.
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NOTE
Wind-fence enclosures of steel and concrete can be utilized. Wind-fence designs can also control reflected light
from the combustion chamber.
6.5.3
The wind-fence design shall provide for an external surface temperature acceptable for worker exposure.
6.5.4
Securable doors or man-ways shall be provided to limit access to the inside of the wind-fence.
6.5.5 The size of the access door shall be sized for inspection, maintenance and repairs. This can be also
achieved through other methods such as removal of wind-fence panels.
6.5.6 Enclosed-flame flares shall also be equipped with at least one viewing port for observation while the flare
is in service. View port(s) may be installed on the wind-fence or on the combustion chamber itself if the design
allows safe access to this location.
NOTE
6.6
The number of doors and view ports is selected based on inspection and access requirements.
Radiation Shielding
6.6.1 All piping subject to thermal radiation, upstream of the individual burner risers, shall be adequately
protected.
NOTE
6.6.2
Such piping is typically covered with loose gravel or metallic shielding.
The covered piping shall be suitably protected from environmental effects and corrosion.
27
Annex A
(informative)
Flare Equipment Overview
FO
A.1 Types of Flares and Components
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A.1.1 General
Refer to Figure A.1 for a general flare type selection guide. Within each general type of flare, various alternatives
and proprietary design aspects can exist.
Flares can be grouped into the major categories described in A.1.2 to A.1.7.
a
Liquid burners and other assist media, such as high-pressure gas or water, are not typically used in refinery and
petrochemical plant services but have been used in production facilities. These technologies are outside the scope of this
Standard. See API Standard 521 for considerations for liquid disposal.
Figure A.1 — Flare Type Selection
28
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publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
A.1.2 Elevated
A.1.2.1
General
FO
Elevated flares are generally oriented to fire vertically upward. The discharge point is at an elevated position
relative to the surrounding grade and/or nearby equipment.
There are several types of support methods for elevated flares, as described in A.1.2.2 to A.1.2.4.
Self-supported
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A.1.2.2
A free-standing flare stack supports the elevated flare-burner without the use of guys or a derrick support. See
Figure A.2 for a typical self-supported structure.
A.1.2.3
Guyed
An elevated flare stack supports the flare-burner through the use of cables. Cables are attached to the flare stack
at one or more elevations to limit the deflection of the structure. The cables (guy-wires) are typically positioned in
a triangular plan to provide strong support. See Figure A.3 for a typical guy-supported structure.
A.1.2.4
Derrick Supported
A steel trussed structure of one of the following types supports one or more flare risers/stacks.
a) Fixed Derrick:
The stack is permanently supported by the structure. The flare-burner can be maintained by lowering it with a
davit or crane. See Figure A.4 for a typical fixed derrick support structure.
b) Demountable Derrick (Multiple-section Riser):
A derrick with the riser(s) mounted to permit lowering the flare-burner to grade for service. The riser is
typically assembled in sections that can be raised and lowered using a track and guide system. This allows
grade-level access to the flare-burner. Many demountable derricks are designed to support multiple risers
with their flare-burners. Such a derrick can be designed so that all the flares, except the one being maintained,
can be operational during the maintenance.
Figure A.5 illustrates a derrick with a multiple-section demountable riser, in the normal operation position
[Figure A.5 a)], with upper sections slightly lifted at the start of the lowering operation[ Figure A.5 b)], during
the process of lowering the bottom section [Figure A.5 c)], and with the bottom section completely lowered
[Figure A.5 d)].
c) Demountable Derrick (with Single-section Riser):
A derrick with a single riser utilizing a track and guide system that allows the lowering of the flare riser and
burner to grade in a single piece.
Figure A.6 illustrates a derrick with a single-section demountable riser: (1) in the normal operating position, (2)
during the lowering process and (3) completely lowered to allow grade level access to the flare-burner.
29
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
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Figure A.2 — Self-supported Structure
30
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
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Figure A.3 — Guyed-support Structure
31
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
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Figure A.4 — Typical Fixed Derrick Support Structure
32
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
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a) in the normal operating position
b) with upper sections slightly lifted at the start of the lowering operation
Figure A.5 — Demountable Derrick with Multiple-section Riser
33
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
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c) during the process of lowering the bottom section
d) with the bottom section completely lowered
Figure A.5 — Demountable Derrick with Multiple-section Riser (continued)
34
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
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Key
1
2
3
in the normal operating position
during the lowering process
completely lowered to allow grade level access to the flare-burner
Figure A.6 — Demountable Derrick with Single-section Riser
A.1.3 Horizontal
The flared liquids and gases are piped to a horizontal flare-burner that discharges into a pit or excavation
(see Figure A.7).
Figure A.7 — Horizontal Flares
35
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
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A.1.4 Enclosed-flame Flares
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Enclosed-flame flares are constructed to conceal the flame from direct view. They can reduce noise and minimize
radiation. Unlike an incinerator, a wide turndown is inherent with these systems. The normal flow rate allows the
connected facility to start-up, shutdown and operate on a day-to-day basis without exposed flame flaring. Multiple
stages within the enclosed flares are sometimes used. Figure A.8 illustrates a horizontally fired enclosed flare.
Either horizontally or vertically fired burners can be used in enclosed-flame flares.
Key
1
2
staging control system
pilot
3
4
wind fence
burner
5
stack
a
Flue gas.
b
Airflow.
Figure A.8 — Enclosed-flame Flare
36
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
A.1.5 Single- and Multi-burner
A.1.5.1
General
FO
Single- and multi-burner flares may be provided with each of the flare categories in A.1.2 to A.1.4.
A.1.5.2
Single-point Flares
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A single-point flare is an open-pipe flare with a single exit point. Single-point flares may be of the smokeless or
non-smokeless design. They are generally vertical types. A single-point flare usually has a lower pressure
differential than a multi-burner flare.
A.1.5.3
Multi-burner Staged Flares
Multi-burner flare systems utilize the available pressure energy of the gas to entrain additional air. This improves
combustion, as the flare gas is better mixed with the air. Multi-burner flares are usually designed to achieve
smokeless combustion if adequate pressure and space are available. The multiple burning points may be
arranged in arrays located near grade or at an elevated position. Figure A.9 illustrates a multi-burner staged flare
located near grade.
Multiple, elevated, single-point flares in a multi-staged arrangement can be provided to accommodate very large
relieving capacities, thereby reducing the size of the individual stacks and the associated burner and reducing
flame-front instability and burnback at low capacities.
See also Annex B.
Key
1
2
3
berm (fence enclosure)
staging control system
burner
Figure A.9 — Multi-burner Staged Flare
37
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
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Figure A.10 illustrates the performance of a three-stage flare system. Each stage adds incremental exit area.
Staging allows operation of each stage at pressures where smoking does not occur. As the flow increases in the
first stage, the system pressure increases. When the system pressure reaches a maximum for the first stage, the
second stage opens providing additional capacity. The additional capacity decreases the system pressure while
maintaining the same flow. This process then repeats with the third stage. Decreasing flow reverses the staging
process. The chart illustrates three steps of operation: step 1 equals 20 %, steps 1 and 2 equal 50 %; steps 1, 2
and 3 equal 100 %. Many variations of staging size, number of stages, pressure and sequence are possible. See
also A.11 and Figure A.17.
Single- and multi-burner staged flares are fed from a manifold. The manifold distributes the flow of flare gas to
individual branches containing one or more flare-burners. ON/OFF valves direct the flow of flare gas to each
branch. The operation is described above. See also 4.5.1c for additional requirements.
Key
X
flow rate
Y
1
percent maximum pressure
first stage
2
3
first and second stages
first, second and third stages
X
Figure A.10 — Multi-burner Flare Staging Curve
A.1.6 Smokeless and Non-smokeless Flares
A.1.6.1
Smokeless Flares
Smokeless flares eliminate any noticeable smoke over a specified range of flows. Smokeless combustion is
achieved by utilizing air, steam, pressure energy, or other means to create turbulence and entrain air within the
flared gas stream.
Local regulations and plant specific requirements generally define smokeless burning. Opacity is defined by the
Ringelmann numbering scale (Ringelmann 1 is 20 % opacity; Ringelmann 0 is clear).
Typically, the smoking tendency is a function of the gas calorific value and of the bonding structure of the
hydrocarbons. The paraffinic series of hydrocarbons has the lowest tendency to produce smoke, whereas olefinic,
diolefinic and aromatic series of hydrocarbons have a much higher tendency to produce smoke.
38
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
Smokeless flares can be provided with a steam-assist or air-assist system to improve combustion. An air-assist
system utilizes fans to provide mixing energy at the flare-burner. See Figure A.11 for a typical arrangement of an
air-assisted flare.
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Gas system hydraulics (i.e. the gas-pressure drop available for the flare equipment) can influence the method
chosen for smoke suppression. The pressure (kinetic energy) of the flare-gas can, if sufficient, be used to make
the flare operate without smoke. The smoke-suppression method is dependent on the utility availability.
Key
1
2
gas discharge ports
gas riser
3
4
flanged inlet for gases being flared
stainless steel flare-burner
5
6
low-pressure air riser
vaneaxial low-pressure air burner
7
8
two-speed motor
inlet bell
39
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
Figure A.11 — Air-assisted Flare
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This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
A.1.6.2
Non-smokeless Flares
FO
Non-smokeless flares utilize no outside methods (air, steam, etc.) to improve combustion. This sometimes results
in the presence of some smoke at certain operating conditions. Non-smokeless flares are typically, but not
exclusively, single-point flares. Non-smokeless flares can be used to supplement smokeless flares when the
capacity on the smokeless flare is exceeded.
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Non-smokeless flares are used for hydrocarbon or vapor streams that do not cause smoking (i.e. methane,
hydrogen, carbon monoxide, clean-coke oven gas, ammonia or hydrogen sulfide) or when smoke is not a
consideration.
A.1.7 Endothermic (Fuel-gas-assisted) Flares
Endothermic (fuel-gas-assisted) flares are used when flaring low-heating-value waste streams. Refer to
th
API Standard 521 6 edition, 5.7.3.4 for guidance on the basis for selection of an endothermic-type flare.
There are several possible arrangements for endothermic flares including:
 simple, non-assisted flare-burner with high-energy supplemental gas added to the relief-gas steam upstream
of the flare;
 simple, non-assisted flare-burner with a pre-mixed supplemental fuel/air mixture supplied to an annulus
surrounding the relief-gas exit;
 single-point flare with an air-assisted supplemental gas burner surrounding the relief-gas exit.
A.1.8 Major Components
A.1.8.1
The major and optional components for an elevated flare are the following:
 flare-burner with or without smoke suppression and control (optional) capability;
 pilot(s);
 pilot igniter(s);
 pilot-flame detectors;
 buoyancy or velocity seal (optional);
 support structures;
 grounding connection;
 knockout drum (optional);
 flame/detonation arrestor (optional);
 liquid seal (optional);
 piping;
 smoke-suppression control system (optional);
 blower(s) (optional);
 flow, composition, heat content or video monitoring (optional);
 ladders (caged or with safety-climbing system) and platforms (optional)(see A1.9);
 davit for burner removal (optional);
41
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
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 aircraft warning lights and painting (optional);
 radiation heat shields (optional);
FO
 rain shields (optional).
A.1.8.2
The major required and optional components for multi-burner flares and flare-burners are similar to
those for elevated flares plus the following:
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 staging equipment and instrumentation (optional),
 heat shielding for gas manifolds and headers (optional),
 fence (type and purpose are site-specific) (optional).
A.1.8.3
The major required and optional components for an enclosed flare are similar to those for
multi-burner flares plus the following:
 enclosure (structure, dike or fence),
 refractory (optional),
 heat shielding for gas manifolds and headers (optional).
A.1.9 Ladders and Platforms
An elevated flare can meet its design intent as stated in 4.1 without being equipped with platforms or ladders. The
following guidelines for provision of ladders and platforms are as follows.
a) For elevated flares more than 15 m (50 ft) high, a 360° platform should be provided near the top. This
elevation may be increased depending on site-specific capabilities and maintenance practices. This platform
should be located below the flare-burner mounting flange or connecting weld and be positioned so that it can
be used during inspections, maintenance and flare-burner replacement.
b) If the flare is equipped with a buoyancy-type air seal, a platform should be provided for access to the
inspection and clean-out nozzles. A 120° platform is commonly used.
c) Access platform(s) should be provided for inspection and maintenance of fixed, non-retractable aircraft
warning lights.
d) Additional access platforms may be located at other locations as necessary. For example, the erection plan
for the flare stack can make a full 360° or partial platform desirable at each stack assembly joint.
e) The maximum distance between platforms should be 9 m (30 ft) or in accordance with local regulations if
access between platforms is via ladders.
f)
If the flare system is equipped with a knockout drum and/or a liquid seal, platforms can be desired for access
to man-ways, level instruments, etc.
g) If the flare is of multi-stage type, the staging valves should be accessible for inspection and maintenance.
h) In general, the flare-burner maintenance platform should have a minimum 0.9 m (3 ft) clearance width from
the flare-burner and its appurtenances. In some cases, placement of the platform and the flare-burner/stack
connection at a somewhat lower elevation can allow the use of a smaller-diameter platform.
42
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
A.2 Flare-burner
A.2.1 Purpose
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The type of flare and configuration is primarily established through consideration and application of the
procedures, practices and recommendations of this Standard. The most critical mechanical component integral to
all flare types is the flare-burner, with which all aspects of safe, reliable and efficient discharge and combustion of
relief gases from the flare system are associated. The integrity and reliability of this component has a direct effect
on the operability and run length between maintenance intervals for the facility.
The flare-burner is designed to provide for the safe discharge of the maximum specified relief-gas flow rates at the
system-allowable pressure drop, as detailed by API Standard 521. The flare-burner mixes fuel and air at velocities,
turbulence and concentration required to establish and maintain proper ignition and stable combustion. The flareburner ignites and combusts vapors discharged for process relief, for plant upset and emergency conditions. This
mechanical device controls the combustion process for the specified relief conditions and produces the desired
destruction/combustion efficiency. The mechanical components of typical flare-burner designs are described in
A.2.2 to A.2.7.
Some flares are subject to government regulations that mandate the flare meet certain operational requirements.
Such operational requirements could include relief-gas exit velocity and/or relief-gas heating value. For the United
States, the Code of Federal Regulations 40 CFR PT 60.18 defines requirements for VOC flares. Consult with the
authority having jurisdiction to determine the specific requirements for a particular flare.
Refer to API Standard 521 for further information on steam requirements for smoke suppression and the ability of
different types of flare-burners to handle liquid-hydrocarbon droplets.
A.2.2 Unassisted Pipe Flare
An unassisted pipe flare is used where smokeless-burning assist is not required. Ignition of the flare flame is by
pilots. The pilots are ignited by a pilot-ignition system.
A properly designed pipe flare-burner should include mechanical components, such as flame retention tabs or
other methods inherent in design, e.g. Coanda effect, to provide a stable flame and prevent blow-off. The ignition
fire from the gas discharge is initially ignited by interaction with the flames of the pilot(s). Once the pilot lights the
flare-stabilizing flame, the flare is expected to maintain flame stability over the operating design range.
For unassisted flares subject to 40 CFR PT 60.18, a relief-gas stream is required to have a minimum calorific
3
value of 8 MJ/Nm (200 Btu/scf). Consult current regulations for additional requirements.
The flame produced by an unassisted pipe flare is a function of the relief-gas composition and the gas exit velocity.
At higher gas velocities, the gas discharge energy pulls combustion air into the flame and produces a shorter,
more erect flame that has greater resistance to wind deflections. At lower gas exit velocities, air is drawn to the
flame primarily by the buoyancy of the heated products of combustion. A buoyant flame is typically softer, longer
and more wind-affected than a flame associated with higher gas exit velocities.
Low gas exit velocities and buoyancy-dominated flames may be employed for successful combustion under lowheating-value relief-gas conditions. High gas exit velocities can be employed for hydrocarbon relief gases of
higher heating value or for relief gases rich in hydrogen.
Flare combustion noise is influenced by gas exit velocity. Increased relief-gas exit velocity can produce greater
combustion turbulence and have higher combustion noise levels. The highest combustion noise levels are
realized when a flare-burner is allowed to operate at a gas exit velocity where combustion instabilities occur.
Combustion instability is defined as a flame that lights, lifts off, goes out and re-ignites in a semi-cyclic mode.
The prime operating considerations with unassisted pipe flares are to safely discharge the relief gas from the
flare-burner within the hydraulic design for the flare system (within the allowable pressure drop and flame
combustion velocity limits) and to ignite and burn the relief gas with the designed flame characteristics.
Wind action at low flaring rates can produce internal burning and/or external flames that remain attached to the
flare-burner. The flare-burner should be designed to withstand the effects of such internal and attached external
43
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
burning. On larger-sized flare-burners, internal refractory linings are sometimes employed to mitigate the thermal
effects of internal burning. Refractory linings reduce the high thermal gradients that produce buckling in flareburners. Buckling of the flare-burner shell is the first sign of almost all flare-burner failures.
FO
Wind shields may be employed on pipe flares to help mitigate wind-induced flame attachment to the external
flare-burner surfaces.
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The sizing and engineered design of pipe flares takes into consideration the full range of relief-gas compositions
and flare-burner exit velocities within the defined hydraulic limits. These considerations ensure proper and safe
pressure relief protection for the operating plant.
A.2.3 Steam-assisted Pipe Flare
The basic flame stabilization for a steam-assisted smokeless pipe flare is similar to that for the basic pipe flare
described in A.2.2. The steam-assist equipment should not disrupt the basic flame-stabilization mechanisms of
the flare-burner and can, in some cases, be made to assist in the flame stabilization.
Gas exit velocities are limited for steam-assisted flare-burners in ways similar to that for basic pipe flares. Steam
injection adds potential dilution to the relief gases, even when steam is operated at minimum purge rate. Steamassisted flares typically require more combustible gas mixtures to achieve desired VOC destruction efficiencies.
For steam-assisted flares subject to 40 CFR PT 60.18, a relief-gas stream is required to have a minimum calorific
3
value of 12 MJ/Nm (300 Btu/scf). Consult current regulations for additional requirements.
The flame produced by a steam-assisted pipe flare is a function of the relief-gas characteristics, the gas exit
velocity and the steam-injection design.
Steam assist is used to control the formation of smoke that accompanies the relief of many hydrocarbon gases.
The considerations for smoking tendency are described in A.1.6.1. The steam injection functions to produce
smokeless combustion by educting combustion air, thus increasing momentum and turbulence in the flare flame.
The addition of combustion air, momentum and turbulence can produce flame characteristics for smokeless flaring
where shorter, more intense flames are produced. These flames have greater resistance to wind deflection and
can have reduced radiation fractions.
The quantity of steam required for smokeless burning is a function of the gas composition, the flare-burner size
and design, the steam-injector design and operating pressure and the environmental conditions. While steamassist enhances the combustion of relief gases that smoke, it adversely affects the combustion of relief gases with
a high level of inerts. Relief gases with a high level of inerts, when flared from a steam-assisted flare, can require
a greater calorific value to sustain the required flame stability and hydrocarbon-destruction efficiency.
Steam is often injected into the relief-gas discharge at the top of a flare-burner. Typically, a steam ring that has a
number of injection nozzles or slots is employed. The design and location of injector nozzles varies as different
flare manufacturers each have their own proprietary design.
Steam consumption varies widely as a function of the particular gas being flared and the manufacturer’s
proprietary design of the flare-burner. API Standard 521 provides guidance on typical steam rates for elevated
flares.
The upper steam injection functions to inspirate air and to force the air mixture into the relief gas discharging from
the flare-burner. The steam-injection pattern is intended to enhance fuel-air mixing and can add to the momentum
of the relief-gas discharge. The steam and air acts to dilute the hydrocarbon fuel content, which also reduces the
smoking tendency. The steam vapor can also participate in the combustion kinetics, assisting in the conversion of
carbon to carbon monoxide.
Compressed air or other high-pressure gases, including gaseous, light-relative-atomic-mass hydrocarbons, can
be used in an upper “steam” ring, but steam has been found to be the most effective medium.
The effective addition of steam from an upper steam ring increases the turbulence of combustion. The overall
noise level of the flare-burner increases due to both the additional combustion noise and jet noise from the steam
44
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publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
nozzles. Steam-assisted, smokeless flares can have significantly increased overall noise levels in comparison to
flares with no steam assist.
FO
In addition to the operating considerations mentioned previously for the pipe flare, attention should be given to the
rate of steam injection. If too little steam is added to the flare-burner, a smoking, softer, more wind-deflected flame
is produced. Proper steam injection proportions the steam-injection rate to the relief-gas flow rate. It is acceptable
to operate the steam injection just above the incipient smoke point for the gas composition and flow rate.
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Higher steam-injection rates make the flame harder, cleaner and less wind-deflected. Higher steam-injection rates
also increase the noise levels. Excessive steam-injection rates produce combustion instability accompanied by
excessive flare noise (low frequency, pulsating). At the extreme, over-steaming can extinguish the flame.
The addition of steam-injection equipment does not change the purge-gas requirements for a flare-burner.
The upper steam ring is subjected to flame impingement due to wind action. A cooling steam flow is utilized to
mitigate this. This minimum steam flow is also set to maintain a suitable temperature in the steam system to avoid
condensation and water-hammer effects in the ring and steam line. The minimum steam flow is a function of the
flare-burner design.
A properly designed upper steam ring can function as a windshield to reduce adverse wind effects on the flare
flame. It can also be used to eliminate external flame attachment to the flare-burner barrel.
Steam flows from an upper steam ring can condense and create water and ice problems for a flare-burner.
Excessive condensate can produce large icicles on flare structures. These icicles pose a hazard to the personnel,
flare system and piping. The use of a defined minimum flow of dry or preferably, slightly superheated steam
together with the proper design and layout of steam trapping facilities are critical to minimizing these problems
A center steam injector may be used to mitigate internal burning. A properly designed center steam injector adds
steam to the low-relief-flow-rate gases. This helps to push the flame out of the flare-burner and lowers the peak
flame temperatures. Center steam is also effective for smokeless burning at low relief-gas flow rates. Center
steam dilutes the fuel hydrocarbon content to avoid smoke generation.
Excessive center steam can produce combustion instability and extremely high noise for low gas flow rates.
Center steam adds water vapor to the inside of the flare stack. The steam can condense, forming water and ice
inside the flare system. The potential for such problems can be reduced by using separate steam risers and
controls for the center steam injector.
CAUTION Care should be taken in design and operations of steam systems in cold environments. The
potential exists to form an ice plug that reduces the hydraulic capacity of the flare to below that needed
for plant safety.
The operation of steam-assisted pipe flares is described in A.2.8.
A.2.4 Pipe Flares with Internal Steam/Air Eductor Tubes
Smokeless flaring at higher rates and lower flare noise can be achieved by injecting steam into the relief-gas
discharge from tubes located inside the flare-burner barrel. These internal tubes are designed to act as
combustion air eductors that use the steam energy to pull in combustion air and to mix the air with the relief gas.
The steam/air discharge out of the internal tubes can also be at a high velocity, adding to the momentum of the
flare discharge and inspirate additional combustion air while stiffening and shortening the flame.
Typically, a reduced steam-to-hydrocarbon ratio is required for internal steam/air tubes. This is because the tubes
increase the effective mixing of the steam and air with the gas. The internal steam/air tubes can enter the flareburner barrel at different elevations. Therefore, combustion air access is not limited to the upper perimeter of the
flare-burner (as it is for an upper steam ring assembly).
Greater access to combustion air increases the maximum achievable rate of smokeless burning. A flare-burner
that employs both internal steam/air tubes and an upper steam ring can have more than twice the maximum
smokeless burning capacity of an upper steam ring flare.
45
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
With the internal steam/air tubes entering the flare barrel at elevations well below the flare-burner discharge point,
the steam injector can be designed for effective air eduction with reduced steam-jet noise levels. Furthermore, at
the lower tube entry location, a muffler assembly can be used to further reduce the steam-injection noise. A
properly designed flare-burner operating only on the internal steam/air tubes can be 10 dB to 12 dB quieter than
one operating with only an upper steam ring.
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There are additional flare operating considerations with internal steam tubes because of the possible
condensation of steam. The use of dry steam is important to avoid condensation and possible freezing. Proper
removal of steam condensate from the internal steam/injection point and from any muffler surrounding the tube
assemblies is especially important in areas where long periods of freezing temperatures are expected to occur.
Back-burning potential is a hazard with steam/air tubes. Care should be taken to avoid back-flow of combustible
mixtures in the internal tubes. The most common cause of back-flow in the tubes is improper flare operation. If the
upper steam ring is pressurized prior to engaging the steam supply to the steam/air tubes, the upper steam can
cap the top of the flare discharge and force flow backward out of the tubes.
Operation of a steam-assisted flare with internal steam/air tubes is described in A.2.8.
A.2.5 Air-assisted Smokeless Flares
Air-assisted flares are used where smokeless burning is required. It is used when steam is not available.
For air-assisted flares subject to 40 CFR PT 60.18, a relief-gas stream is required to have a minimum calorific
3
value of 12 MJ/Nm (300 Btu/scf). Consult current regulations for additional requirements
Air-assisted flares often employ gas distributors to promote mixing of the relief gas with a low-pressure blowerdelivered forced airflow. The gas-flow distribution arrangement more closely resembles traditional burner designs
than pipe flares. The gas-air distributors/mixers use the burner-type flame-stabilization mechanisms. These
include flame retention devices and aerodynamic-type flame-stabilization methods.
The flame produced by a low-pressure, air-assisted flare-burner is a function of the combined mixing energy of the
relief-gas discharge and the forced airflow rate and velocity. Typically, the low-pressure air blower delivers only a
fraction of the flow rate of air required for stoichiometric smokeless combustion. This air fraction is used to
promote mixing with the relief-gas discharge and to add momentum to the flare discharge to effectively entrain
additional combustion air from the surrounding atmosphere.
The flame produced by an air-assisted burner can be shaped by the use of the forced-draft air. The flame can be
developed in an axial airflow manner to produce an erect, vertical flame. Alternatively, the forced air can be
swirled to promote a rotational airflow that can produce a wider, shorter flame.
The rate of smokeless burning and the flame characteristics are somewhat adjustable by the quantity of
combustion air used and by its energy of discharge (to promote fuel-air mixing and flare-discharge momentum).
The height of an air-assisted, smokeless flare should be designed for the limiting case when the flare can be
required to operate without an air assist. This case can produce the greatest flare radiation. See API Standard
521 for more information regarding flare radiation.
The prime operating considerations with an air-assisted flare are to safely discharge the relief gas from the flareburner within the hydraulic design for the flare system (within the allowable pressure drop and flame combustion
velocity limits) and to ignite and burn the relief gas with the designed flame characteristics.
Smokeless burning is achieved with a forced-draft air supply. The quantity and velocity of the forced airflow can
be proportioned to the gas flow by a blower damper, blower speed control or other means. Alternately, the forced
airflow can also be controlled in discrete steps by the use of multiple-speed blowers or multiple blowers. At low
relief-gas flow rates, a minimum, continuous airflow can maintain a cooling airflow and proper aerodynamic design
across the burner. Airflow can be increased as the relief-gas flow rate increases. Care should be taken not to
over-aerate the flame. Over-aeration can produce combustion instabilities that increase flare noise and vibration.
At an extreme, excessive assist airflow can extinguish the flame.
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This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
The purge rate of the air-assisted flare should take into consideration the flare-burner design, size and the forced
airflow interaction with the wind and environmental factors at turndown conditions. A minimum airflow rate is
required to protect the spider arms or internals of the burner from overheating.
FO
A blower system should be designed to produce the design airflow rate and velocity at the flare-burner
considering the air delivery system. The blower power should be selected with regard to the delivery of the
densest air (coldest ambient temperature). Blowers of all types, including axial, centrifugal, etc., have been used
for air-assisted smokeless flares.
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A.2.6 High-pressure Smokeless Flares
High-pressure smokeless flares are used where smokeless burning is required and the relief gases are
discharged from the flare-burner at a high velocity. The pressure required is dependent on the gas composition,
burner design and other factors. The pressure of the relieving gas is converted to kinetic energy to promote air
entrainment and mixing, which produces smokeless burning. The advantage is that supplemental energy from a
steam supply or a forced-draft blower can be eliminated or minimized.
High-pressure flare flames can be stabilized using the aerodynamic effects of the relief-gas discharge and its
entrained air. Mechanical flame-holding devices are often not required when this aerodynamic effect is used in the
flare-burner design, e.g. Coanda flare, convergent jet nozzles, etc. As high-pressure flares operate with high gas
exit velocities, it is necessary that the gas compositions flared with this equipment be rich in hydrocarbon fuel
and/or hydrogen. High-pressure flare technology should not be used for cases involving combustion-stabilitylimited relief-gases such as those containing a high inert content. For cases when the relief-gas contains sufficient
hydrocarbon fuel, high-pressure flares have been shown to produce very high hydrocarbon-destruction
efficiencies, exceeding 98 %.
The flame produced by a properly designed and operated high-pressure flare-burner effectively converts the gas
pressure to kinetic energy that entrains and mixes combustion air with the fuel to produce a smokeless flame that
is resistant to wind deflection. Proper use of the gas pressure energy requires that the gas kinetic energy have the
opportunity to entrain combustion air. For some gas flow rates and composition, this requires the use of multiple
nozzles or gas distributors.
If a flare-burner is designed for high-pressure operation, there can be some turndown gas flow rate at which the
pressure conversion to gas kinetic energy is insufficient to properly entrain and mix combustion air. At this
turndown condition, the high-pressure flare can have a smoking flame that is subject to wind deflections. To
improve turndown, high-pressure flares are often staged to promote smokeless burning with design flame
characteristics over a wider operating range (multi-burner flare systems).
High-pressure flare technology is particularly effective for oil and gas production facilities. High-pressure flares
can greatly reduce the flare flame radiation.
The prime operating considerations for high-pressure flares are to safely discharge the relief gas from the flareburner within the hydraulic design for the flare system (within the allowable pressure drop and flame combustion
velocity limits) and to ignite and burn the relief gas with the designed flame characteristics. When multiple nozzles
are used, location of these nozzles is important to ensure proper ignition for the gas discharge from each nozzle.
Gas compositions and/or flow rates that are not compatible with the high-pressure flare design should be avoided
in design.
High-pressure flare operations produce both combustion noise (due to the high-intensity flame produced) and jet
noise due to the high-pressure discharge of the relief gas to the atmosphere. With proper high-pressure flare
design, the combined flare noise level can be managed. Overall noise levels for a properly designed highpressure flare might not exceed the noise level of a comparably rated steam-assisted smokeless flare.
High-pressure flare operations produce flames that are dominated by the kinetic energy of the gas discharge.
These flames have significant upward momentum and are not unduly affected by cross winds. High-pressure
flare-burners, though, should be designed to accommodate any flame impingement that can possibly occur at low
relief-gas flow rates and, thus, at low-pressure operation.
NOTE
Other assist media for smokeless combustion are available for flare-burners. However, they are not typically used
in refinery or petrochemical services. Such media include high-pressure gas and water for liquid and vapor combustion. These
technologies are applicable to offshore production facilities and are not included in this standard.
47
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
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The pressure drops and high velocities associated with high-pressure flare operations can produce loading
conditions usually not expected in low-pressure flares. Examples are jet forces and vibrations originated by
process turbulence or operative modes that can cause acoustic-fatigue failures on mechanical details of the flare.
The components typically subject to such conditions include the flare-burner, the mechanical attachments to the
flare-burner and other components in the flare-gas line, including any purge-gas reduction seals. The process and
mechanical design for high-pressure flares with exit velocity greater than 0.8 Mach should be performed in order
to avoid or prevent acoustic-fatigue failures. Fabrication should follow the same approach. The pressure design
code can be used by designers to support the mechanical design and manufacture of flares under vibration and
cyclic conditions. An important aspect of construction is the attention to weld details. For example, welds should
be the full-penetration type where butt welds are preferred.
Among the possible sources of vibrations for consideration, the most evident in high-pressure flares include
combustion noise and flaring-gas pressure pulsations. The following may be used as guidance to the designer if
no other information on the above sources is available.
a) Combustion noise:
The maximum combustion noise can be assumed in accordance with Table A.1 (case including smokeless
steam) to be properly transformed in pressure waves loading the exposed top sections of the flare-burner to
the location of analysis.
Table A.1 — Combustion Noise Spectrum
Frequency, Hz
63
125
250
500
1000
2000
4000
8000
Loudness, dB
95
101
109
118
115
112
110
107
Total, dBA
120
b) Gas pressure pulsation:
The gas pressure pulsation, P, is the internal pressure pulsation derived by process (flaring gas) turbulence
where its most significant dynamic load can be assumed, as a first approximation, to be a sinusoidal load as
given by Equation (A.1):
P  P0  sin  2 ft 
(A.1)
where
P0 is the pressure-pulsation amplitude from top peak to bottom peak; if no data exist, it can be assumed to
be between 1 % and 5 % of the local static pressure;
f
is the exciting frequency range, evaluated between 0 Hz and 200 Hz;
t
is the time, expressed in seconds.
In addition to the contributions of noise and pulsation from the flaring gas, other sources that can contribute to the
overall vibration include
 internal discontinuities in the process gas piping, which cause vortex shedding and turbulence;
 liquid-droplet entrainment;
 fans;
 cyclic operation of the blow-down system.
48
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
A.2.7 Mechanical Details of Flare-burners
A.2.7.1
Flare-burner Dimensions and Connections
FO
Flare-burners are typically dimensioned from their attachment point at the flare support structure to the gas
discharge point.
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Flare-burners are often nominally described by their connecting pipe size, i.e. a DN 600 (NPS 24) flare-burner for
a DN 600 (NPS 24) pipe riser, a DN 1500 (NPS 60) flare-burner for a DN 1500 (NPS 60) pipe riser. The upper
diameter of a flare-burner can be larger than the nominal diameter to accommodate internals such as steam/air
eductors.
Elevated pipe flares with or without steam-assist nozzles are either flanged to the flare riser or can be welded.
Flare-burners sized DN 100 (NPS 4) or smaller may be attached by threaded connections. Steam supply
connections should be forged flanges of a rating appropriate for the steam service connection. Pilot gas and
flame-front generator (FFG) connections sized DN 25 (NPS 1) or less may be threaded, flanged, socket welded,
or butt-welded.
Internal gas connections on air-assisted flare-burners are often welded to avoid obstructing the airflow.
High-pressure flare attachments should be designed with a consideration of the thrust load produced from the gas
discharge and in accordance with the principles of the design pressure code. Material thicknesses for the flareburner should be suitable for the service and should be clearly indicated on the data sheets and drawings.
A.2.7.2
Flange Ratings
For gas risers to flare-burner connections, flanges are normally used. Mechanical suitability should be confirmed
for the specific application. Refer to 5.3 for flange selections, including flange bolting dimensions to be used for
flare-burners larger than DN 600 (NPS 24).
The purchaser needs to specify if other flange standards are to be applied, e.g. ASME B16.47 or EN 1092-1.
The attaching flange should meet the metallurgical requirements of the flare support structure. For a carbon steel
stack a carbon steel burner flange is acceptable.
Flanges may be forged or fabricated from plate of an appropriate material.
A.2.7.3
Flare-burner Handling and Lifting Lugs
Flare-burners are often equipped with lifting lugs or brackets for initial attachment. These lugs or brackets should
be removed prior to placing the flare-burner in service. The lug or bracket, if left attached, will be subjected to the
flare operating conditions, including possible internal or external flame attachment. The initial installation bracket
should not be trusted for removal of a flare-burner after it has been placed in service. Often, such lifting lugs are
made of carbon steel and designed to “burn off” in operation.
A.2.7.4
Materials
It is necessary that flare-burner components have an acceptable fatigue and elevated-temperature strength. It can
be necessary that they be resistant to thermal cycling, stress-corrosion cracking, high-temperature corrosion (in
reducing or oxidizing atmospheres) or ambient-temperature corrosion. Flare-burner materials normally consist of
austenitic stainless steel or high-nickel alloy depending on the particular service.
The upper section of the flare-burner, whether or not it is steam-assisted, is typically fabricated of heat-resistant
alloys. Grade 310 stainless steel is a normal standard. Alternative materials for greater heat resistance or to
improve resistance to fuel and combustion product corrosion and erosion can be supplied. The lower flare-burner
section material, including the attachment flange, can be fabricated of carbon steel or lower-grade 304 stainless
steel where flare process conditions do not warrant alternative materials for temperature, corrosion, erosion or
other factors.
49
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
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the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
Air-assisted flare-burners may use shorter lengths of 310 stainless steel and/or alternative materials as the flareburner is normally cooled by the forced airflow.
High-pressure flare-burner material is a function of the flare-burner design and flame impingement that can occur
at low rates.
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It should be noted that many flare-burner material failures occur at low relief-gas flow rates. At these low rates, the
flare-burner is most subjected to the detrimental effects of internal or external attached flames. At higher gas flow
rates in a pipe flare, an air-assisted flare or a high-pressure flare-burner, the flame is lifted out of and off of the
flare body. In fact, at high flow rates, the convective effects of the gas flow effectively cool the flare body.
The material of the upper steam ring and ejectors should be selected on the basis of exposure to the flame and
cyclic conditions. The material selection for the flare-burner steam piping should take into account the effects of
intergranular corrosion due to the wet, cyclic, high-temperature service.
A.2.7.5
Welding Requirements
Flare-burner barrel and welded attachments should be welded in accordance with the applicable pressure design
and/or structural design welding requirements.
A.2.7.6
Hydro-testing for Flare-burners
Hydro-testing for flare-burners is not required nor is it recommended.
A.2.7.7
Attachments to Flare-burner
Any piping load applied to the flare-burner connections should be clearly defined. Thermal load, dynamic loads
and dead loads (including the mass of water in the steam lines) should be considered. Only auxiliary piping
associated with the flare-burner should be supported off of the flare-burner. The attachment of pilots, steaminjection equipment, windshields and the like, should accommodate the differential thermal growth that can occur
during service.
A.2.7.8
Windshields for Flare-burners
Windshields are applied to unassisted pipe flares. A properly designed steam-assisted flare, air-assisted flare or
high-pressure flare-burner should not require a windshield. Windshield design is somewhat proprietary to the
flare-burner manufacturer. As a basic consideration, windshields for pipe flares should be considered sacrificial
equipment. The windshield is likely to burn up, sacrificing itself to promote improved service life of the pipe-flareburner.
The design of a windshield located above the gas exit of the burner can require special design considerations.
A.2.7.9
Mufflers for Flare-burners
Mufflers for flare-burners should be designed to prevent damage from excessive flame conditions. Muffler design
should not restrict the flow of combustion air into the flare flame. Mufflers for lower steam-/air-injection locations
are effective in reducing steam-jet noise. These mufflers, if properly designed, improve airflow into the internal
steam/air tubes by mitigating wind effects. Mufflers, if required for air-assisted flares, are typically applied to the
forced-draft blower air intakes.
Noise from high-pressure flares is controlled by proper nozzle design factors.
A.2.7.10 Refractory for Flare-burners
Refractory (internal or external) should not be used except in special circumstances. If used, it is typically for
large-diameter [greater than DN 1050 (NPS 42)] pipe flares to mitigate the effects of internal burning. External
refractory has been used to protect the flare-burner barrel from external burning on the downwind side of the flare.
It is necessary that the refractory used for flare-burners be temperature- and thermal-shock resistant. Flareburners experience rapid changes in temperature. Internal refractory should be well anchored and it is
50
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publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
recommended that the refractory material include metal needles to help hold it in place. External refractory linings
are similar in application.
FO
With any refractory installation, it is necessary to consider the consequences of a refractory failure. For dense
refractory material, this consideration focuses on where the failed refractory will fall. The refractory can fall inside
the flare and potentially obstruct the relief-gas flow. External refractory falling to grade can limit potential access to
the base of the flare structure.
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If a lined burner is specified, experience has shown that no single refractory material or attachment method is
suitable for all cases. In fact, vendors are continually modifying their specifications based on user experience and
material improvements. Therefore, when approving a lining specification, attention should be paid to the following:
a) refractory material: selection in consideration of the refractory product service temperature range, its optimum
thickness, method of installation and its susceptibility to moisture;
b) attachment method: there are various methods of anchoring refractory; typically “bull horns” or hexmesh are
used; consideration should be given to the installation procedure and the potential for the anchoring system to
create shear planes in the lining;
c) refractory reinforcement: this is typically in the form of stainless steel needles; the needles reduce cracking
and hold the refractory together;
d) refractory installation: installation is in accordance with the refractory manufacturer's recommended
installation procedures by qualified personnel;
e) refractory curing and dryout: the refractory manufacturer’s recommendations for proper curing and dryout of
the selected refractory product are required to ensure that the published property values are achieved.
Proper steam-assisted flares can use center steam to eliminate the requirement for internal refractory linings; airassisted and/or high-pressure flare-burners are not typically refractory-lined.
A.2.7.11 Maintenance Issues
Flare-burners generally should be removed when maintenance is performed. All auxiliary-piping connections
should be designed to facilitate flare-burner removal. In some cases, a complete spare flare-burner is kept to
replace flare-burners that are undergoing maintenance.
A portable crane is normally used to remove and replace the flare-burner. In remote locations where cranes of
sufficient height are not available, consideration should be given to providing a retractable davit on the flare
structure. In normal operations, the davit is lowered below the level of the top platform or below the gas seal, to a
position where the flame does not affect it. Lifting tackle should be provided to raise the davit into the lifting
position.
It is necessary to give consideration to the additional wind loads resulting from the flare-mounted davit.
A.2.8 Operations
A.2.8.1
Steam control for a flare-burner equipped with both an upper steam ring and center steam is typically
controlled in the following manner.
a) Set the center steam manually to effectively mitigate internal burning and to produce the desired smokeless
burning for normal, daily, minimum relief-gas flow rates. Avoid excessive center steam quantity, as it creates
excessive flare noise.
b) Operate the upper steam ring to control other relief-gas flow rates, and to produce smokeless burning and the
desired flame characteristics. Excessive upper steam also produces excessive flare noise.
A.2.8.2
A flare-burner equipped with internal steam/air tubes and upper steam ring and center steam rings is
typically controlled in the following manner.
51
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a) Set the center steam manually to effectively mitigate internal burning and to produce the desired smokeless
burning for normal, daily, minimum relief-gas flow rates. Avoid excessive center steam. It creates excessive
flare noise.
FO
b) Allow the internal steam/air tubes to operate before adding steam to the upper steam ring. Adjust the
steam/air to produce the desired smokeless burning rate at the lowest possible noise level.
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c) Start the upper steam ring to increase the smokeless burning rate above that achievable with the internal
steam/air tubes. The upper steam ring may also be used to act as a windshield to reduce wind effects on the
flare flame and on the flare-burner.
A.2.8.3
For pipe-flare operations, the only plant control is to maintain flare pilot ignition and maintain proper
flare purge rates.
A.3 Pilots
A.3.1 Purpose
The flare pilot is expected to reliably ignite the flare-burner and maintain stable combustion throughout the full
range of process conditions, including under severe weather conditions, without the requirement for maintenance
for at least five years of operation, unless the pilot is accessible for on-stream maintenance. It is recognized that
in some extreme services, such as burn-pit flares, this five-year lifetime might be unachievable.
If the pilot fails, unburned hydrocarbons and/or toxic gases can be released into the atmosphere, potentially
resulting in a vapor-cloud explosion, odor problems or adverse health effects. In most elevated-flare applications,
the pilot cannot be accessed for service or replacement while the flare is in operation. In order to improve pilot
operability and reliability, as well as provide a means to test individual pilots while the flare is in operation,
individual fuel-supply lines should be installed to each pilot. The pilot system should be reliable enough to operate
for years without maintenance.
A.3.2 General Description
A multitude of pilot designs exists. The majority can be described as fixed heat release, self-inspirating, pre-mix
burners. The principle advantages of such pilots are:
 that the pilot reliability is dependent on only one utility (i.e. fuel gas) since the air is self-inspirated,
 that the pre-mix type design affords greater stability and reliability relative to raw gas or diffusion-flame-type
burners.
In some cases, compressed-air pre-mix pilots have been used instead of the inspirating type. In addition to the
fuel gas supply, compressed-air pilots are also dependent on the reliability of the compressed-air system. To
safely utilize compressed-air pilots, an effort should be made to ensure that the installation includes one of the
following functions.
a) The compressed-air system has sufficient reliability. The probability of pilot failure due to air-supply failure is
acceptably low.
b) The flare can be immediately taken out of service if the compressed-air system fails.
c) The pilot’s function automatically reverts to air-inspiration if the compressed-air supply fails.
d) An independent set of inspirating pilots is installed as backup.
In some cases, direct ignition of the flare or of a slipstream of the flare gas has been used in lieu of a continuous
pilot. Considerations for the use of direct ignition include the following.
a) Without an independent fuel supply, it may not be possible to ensure that a flammable mixture always exists
at the location of the spark.
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This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
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the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
b) Regulations or permitting requirements may require a continuous pilot as a means of ensuring flare gas is
always ignited.
c) The risk of having unburnt combustibles near ignition sources.
FO
d) In some situations, continuous pilots are required to promote the stability of the flare flame.
NOTE
See D.3.3 for additional offshore considerations and hazards.
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In order to ensure stable operation and ignition of the flare gas, the recommended minimum pilot heat release is
13.2 kW (45,000 Btu/h) when flaring hydrocarbon gases with a lower heating value of 12 MJ/Nm3 (300 Btu/scf) or
greater. Pilot heat release in common practice ranges from this minimum up to 102.5 kW (350,000 Btu/h). Pilots
and pilot systems should be capable of remaining lit and continue to ignite the flare under adverse weather
conditions of high wind and rain. Refer to 4.7 for performance and testing expectation and requirements for pilots
and pilot systems.
The number of pilots required is a function of the flare-burner diameter. For very small flares, a single 13.2 kW
(45,000 Btu/h) pilot reliably lights the flare gas. However, it should be noted that if only a single pilot is used, a
single pilot failure represents a complete failure of the ignition system.
As the flare-burner diameter increases, the number of pilots required to reliably light the flare, regardless of wind
direction, increases. The minimum number of pilots recommended for most flare-burners is given in Table 1 as a
function of burner outlet diameter (not hydraulic diameter) when flaring hydrocarbon gases with a lower heating
value of 12 MJ/Nm3 (300 Btu/scf) or greater.
3
For non-hydrocarbon gases or hydrocarbon/inert mixtures with heating values less than 12 MJ/Nm (300 Btu/scf),
additional pilots, higher heat release pilots, or some form of fuel gas addition may be required.
While the recommended minimum number of pilots for flare-burners of 200 mm (8 in.) or less is only one, greater
reliability can be achieved if at least two pilots are installed on every flare. Pilots in excess of those shown are
often added to further reduce the risk of an unburned release.
NOTE
Other means of direct ignition of the flare flame are available other than a continuous pilot; however,
they are not recommended for use in refinery or petrochemical services but may be suitable for offshore facilities.
Such technology includes direct electrical ignition and pellet pyrotechnical ignition systems and may be used in
non-continuous flare systems that include flare-gas recovery. Reasons for not using direct ignition systems on
refinery/petrochemical facilities include:
a) The flare gas may not be suitable for ignition by other than a direct flame.
b) Such a system cannot ignite a flare in the high wind/rain requirements for API compliant pilots.
c) It is not possible to monitor the presence of a flame which can lead to the release of unburned gases.
A.3.3 Mechanical Details
The continuous pilots listed above can be divided into two groups depending on the means of achieving the
fuel/air pre-mix: self-inspirating and compressed air. Both self-inspirating and compressed-air pilots generally
consist of the following components:
 fuel orifice that meters the fuel,
 mixer in which the air and fuel are blended,
 piping that connects the mixer and pilot tip,
 pilot tip where the flame is stabilized,
 fuel regulator (at grade).
53
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the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
The most significant difference between self-inspirating and compressed-air pilots is the design of the mixer. In a
self-inspirated pilot, the mixer is an eductor and the fuel orifice not only is used for fuel metering but is also the
means by which a fuel jet, which inspirates air into the eductor, is created. The components of a typical selfinspirating pilot are shown in Figure A.12.
FO
In a compressed-air pilot the mixer is designed to meter and blend two pressurized gas streams. Two orifices are
required in this case, one for the fuel gas and one for the air.
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This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
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Key
1
2
3
4
5
6
7
pilot tip
pilot mounting brackets
pilot mixer assembly
pilot-gas orifice
strainer
pilot-gas inlet
from flame-front generator
Figure A.12 — Flare Pilot Assembly
55
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
FO
The pilot should be long enough that the mixer is not exposed to the flare flame. The mixer should never become
enveloped in burned or unburned flare gas. The pilot mixer should be a distance of at least 1.8 m (6 ft) or 125 %
of the actual flare-burner diameter (whichever is greater) from the top of the burner. Cast iron, ductile iron and
carbon steel are adequate materials for construction for the pilot mixer, pilot-gas orifice and strainer. Stainless
steel is sometimes used for the mixer in order to avoid deterioration due to rust. Rust can affect the pilot reliability
by blocking the mixer, thereby affecting the air-to-fuel mixture ratio of the pilot.
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Pilots are available with mixers located at much greater distances from the flare-burner and, in some cases, even
at grade. Inspirating pilots of extended length are often made from larger piping. They have few bends in order to
minimize pressure drop.
Because of the increased piping-pressure drop, less pressure drop is available at the pilot tip. Consequently,
these designs often have a more restricted range of gas pressure and composition over which they can operate.
Issues arising from increased pressure drop in extended-length pilots are not as problematic in compressed-air
pilots. Compressed-air pilots can operate at higher pressures.
The pilot tip is continuously exposed to the pilot flame and can routinely be exposed to the flare flame. The pilot
tip should be constructed of a heat-resistant material, such as type 309 SS, type 310 SS, CK 20 or a nickel-based
alloy such as 800H. If the flare or pilot gases are expected to contain H 2S, nickel-based alloys should be avoided
or some protective material should be installed to prevent high-temperature corrosion in this environment.
Weld attachments to the pilot tip should be minimized. Welds on cast pilot tips are prone to cracking, which can
compromise proper operation, ignition or detection of the pilot. Threaded connections on pilot assemblies should
be minimized, in particular at the pilot tip, since they often result in thermal fatigue failure.
The piping between the pilot tip and mixer can occasionally experience exposure to the flare flame. For this
portion of the pilot, an austenitic stainless steel, such as type 304 SS or type 316 SS, is adequate.
In order to minimize the chance of pilot failure due to orifice plugging, a strainer or a settling chamber should be
installed just prior to the fuel orifice. The strainer should contain a screen or wire mesh with openings that are
25 % or less than the diameter of the fuel orifice.
A settling chamber should be sized to remove similarly sized particles. The strainer or settling chamber should be
accessible from grade or from a platform below the flare-burner. Occasionally, a pilot is designed such that it is
completely retractable. In the case of a retractable pilot, not only the strainer, but also the entire pilot can be
serviced at grade.
With a typical-length pilot, the strainer or settling chamber, located just upstream of the fuel orifice, is very near
the flare-burner. It is accessible only when the flare is shut down. In some instances, this strainer has been
removed since it cannot be routinely serviced. A strainer can collect many particles before it plugs, while the pilot
orifice requires only one particle to plug. Strainer removal is not recommended.
Retractable pilots are commonly used on enclosed flares. Retractable pilots are also used in gas plants and
petroleum production where more frequent pilot maintenance might be required due to liquid carry over or
corrosive service from H2S and where crane or other access to the flare tip and pilots is not available. Consult
manufacturers for application/details of retractable pilot systems.
In order to prevent the strainer at the pilot from plugging, an additional strainer should be mounted at grade. The
strainer at grade should allow routine online cleaning. The strainer mounted at grade should be equipped with a
screen or wire mesh that has openings the same size as or smaller than the screen installed in the strainer at the
pilot.
In addition to the strainers mounted at grade, a knockout pot is recommended if there is any possibility for
condensate to form in the pilot fuel line. The pilot fuel supply should also be equipped with its own regulator to
prevent other intermittent gas uses to cause a significant change in pilot gas pressure. The regulator should be
installed downstream of the strainer and knockout pot. A pressure gauge downstream of the regulator is
necessary to properly set the regulator. A flow meter in the pilot fuel gas lines is not a requirement, but can be an
extremely helpful troubleshooting tool when pilot troubles occur.
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the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
In order to accommodate pilot removals for maintenance or replacement during shutdowns, provisions should be
included in the fuel supply for either a double block-and-bleed valve arrangement or the insertion of a blind flange.
FO
If the strainers and knockout pot installed at grade function correctly, the remaining major source of debris that
can plug the strainer upstream of the pilot is corrosion from the fuel-line piping. The most common pilot fuel piping
is DN 15, DN 20 or DN 25 (NPS ½, NPS ¾ or NPS 1) carbon steel piping. This piping is structurally sound.
Unfortunately, carbon steel is subject to corrosion.
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In order to avoid corrosion, stainless steel piping or tubing can be used. Stainless steel piping is the best
alternative, but it is expensive. Stainless steel tubing offers superior corrosion resistance, but has limited durability
if not properly supported. The choice of piping material is left to the user. However, if carbon steel is used with
corrosive plant fuel gas, separate fuel lines to each pilot should be considered. Fuel lines in older installations
should be inspected periodically to determine the condition of the fuel lines. Individual fuel-supply lines may be
installed to each pilot, if required, to improve operability and reliability of the pilots.
A.3.4 Operation
In order to maximize pilot reliability, the most consistent and reliable fuel source should be used. Where possible,
natural gas should be used. Its availability and composition are generally more consistent than that of plant gas.
The likelihood of corrosion associated with impurities is much lower. If plant gas is used, due consideration shall
be given to the range of fuels being handled. Some level of stability under adverse conditions is sacrificed as the
range of fuel compositions being handled is increased.
Prior to operation, the pilot fuel lines should be blown clear. The pilots should not be attached to avoid blowing
debris into the pilot’s strainer or mixing orifice. The composition of gas within the flare system should be confirmed
to be outside the explosive range prior to pilot ignition. Typically, this is achieved by purging with several system
volumes of inert gas prior to pilot ignition. See API Standard 521 for more information on flare purge requirements
prior to start-up.
Once the lines have been blown clear and the flare system has been confirmed to be outside the explosive range,
the pilots can be ignited. Operation of the ignition equipment is covered in A.4. Once ignited, the pilots should be
monitored to confirm that each pilot has a flame. Flame detection is covered in A.5. If the pilots are extinguished,
they should be re-ignited immediately. The pilots should remain lit as long as the flare is in service.
A.3.5 Maintenance
Routine maintenance of the pilots should be performed while the flare is in service. This consists of monitoring the
supply pressure and cleaning the fuel strainers, knockout pots and drains that are accessible from grade.
When the flare is taken out of service, the strainer upstream of the orifice should be cleaned and the orifice should
be inspected. In addition, the pilot tip should be inspected. If the pilot tip shows signs of deterioration, it should be
replaced.
A.3.6 Troubleshooting
The determination of whether a pilot flame has failed is covered in A.5. If a pilot is known to have failed and will
not re-ignite, the explanations in Table A.2 are possible.
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This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
Table A.2 — Troubleshooting of Pilots
Problem
Possible Cause
—
FO
Ignition system
failure.
Plugged pilot tip This can occur at start-up due to debris left behind during
or educator.
manufacture.
Corrective Action
See Clause A.4 before attempting to
troubleshoot the pilots themselves.
Remove debris either manually or via
high-pressure blowing.
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Plugging causes the mixture at the pilot to be fuel-rich. If the
flame does ignite, it is likely to be orange and lazy. If the
plugging is severe, most of the gas can exit the mixer. Severe
plugging can result in a flame exiting from the vicinity of the
mixer.
Plugged pilot tip Debris accumulation while out of service such as a wasp
or educator.
nest.
Remove debris either manually or via
high-pressure blowing.
Plugged pilot tip Unsaturated fuel hydrocarbons.
or educator.
Remove debris either manually or via
high-pressure blowing. Return to
design fuel gas.
Damaged pilot
tip.
If the pilot tip opening(s) have increased in size, the pressure
drop in the pilot will have decreased. The air/fuel mixture at
the pilot will become more fuel lean. The pilot might not stay
ignited or flashback can occur. The pilot can be difficult to
light.
Replace pilot tip.
Plugged
strainer,
plugged nozzle
or plugged
orifice.
This can be detected by turning the fuel gas on and then off. Clean strainer, nozzle or orifice as
If the fuel line is not plugged, the fuel pressure should fall very required.
rapidly. If the fuel pressure does not fall, or falls slowly, then
the fuel line is probably plugged. The flare vendor can advise
as to the time expected for the pressure to fall.
Incorrect fuel.
This can be determined by a fuel sample analysis. If the
hydrogen concentration has increased significantly,
flashbacks may be audible and flames may be visible at the
mixer.
Return to design fuel gas or modify
pilot to match the new fuel
composition. Pilot modifications can
include the following:
a) replacement of the pilot orifice,
b)
c)
adjustment of the air door (if any),
replacement of the pilot entirely.
A.4 Ignition Equipment
A.4.1 Purpose
Ignition equipment is expected to reliably ignite the pilot.
A.4.2 General Description
A.4.2.1
There are four types of ignition systems that are commonly employed to light flare pilots:
 spark ignition at pilot tip,
 spark ignition of a portion of the pilot gas/air mixture prior to the pilot tip,
 compressed-air flame-front generator,
 inspirating flame-front generator.
58
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
No single ignition system is preferred in all circumstances. For improved reliability, multiple ignition systems can
be installed. Spark ignition at the pilot is often preferred as the primary means of ignition because it is easily
automated. A manual compressed-air flame-front generator is commonly installed as a backup system because of
its ultimate reliability and serviceability. Each type of ignition system is described in A.4.2.2 to A.4.2.5
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A.4.2.2
Spark ignition at pilot tip: Spark ignition of a flare pilot at the pilot tip is simple and is easy to automate.
With this system, the spark generation is located somewhere near the pilot tip where it is exposed to the
flammable mixture that exits the tip. In some cases, the life of the sparking device can be shortened due to
continuous exposure to the pilot or flare flame once ignition is established. Unlike pilots used in boilers or process
heaters, the flare pilot or sparking device cannot be replaced while the flare is in operation. Consequently, spark
ignition at the pilot tip is generally not recommended as the only means of pilot ignition.
A.4.2.3
Spark ignition of a portion of the pilot gas/air mixture prior to the pilot tip: In order to limit the sparking
device’s flame exposure, the sparking device can be used to ignite a portion of the pilot’s gas/air mixture prior to
the flame exiting the tip. Downstream of the sparking device, the ignited portion of the gas/air mixture is
reintroduced to the pilot tip. It is necessary that such a system be carefully designed to prevent flashback or stable
burning between the ignition source and the pilot tip. As with spark ignition at the pilot tip, this type of system is
relatively easy to automate. However, the sparking device is still located at the pilot and cannot be serviced while
the flare is in operation. Consequently, spark ignition prior to the pilot tip is generally not recommended as the
only means of pilot ignition.
A.4.2.4
Compressed-air flame-front generator: The most prevalent flare pilot ignition system is the
compressed-air flame-front generator. With this system, compressed air and fuel are metered through orifices into
a mixing chamber located at grade. Downstream of the mixing chamber there is a sparking device and piping
which connects the mixing chamber and sparking device to the pilot tip. See Figure A.13 and Figure A.14. During
operation the flow of combustible gas is established and then ignited. This sends a flame front through the
connecting piping to the pilot tip. The flame front ignites the pilot. The principal advantage of the compressed-air
flame-front generator is that the flow controls and the sparking device are at grade and can be serviced while the
flare is in operation. The principal disadvantage of the flame-front generator is its propensity to form moisture
within the piping leading to the pilot. The moisture can cause corrosion and, if not drained prior to use, can
extinguish the flame front. In cold environments, the moisture problem often results in the requirement for heat
tracing and insulation in order to avoid freezing. Flushing each ignition line after use can reduce the moisture
problem.
A.4.2.5
Self-inspirating flame-front generator: This approach is similar to that of the compressed-air flamefront generator, but instead of compressed air, fuel pressure is used to inspirate the combustion air. The obvious
advantage is that compressed air is not required. The disadvantage is that self-inspirated devices can generate
only limited pressure; consequently, they can be applied only to limited distances and piping configurations.
A.4.3 Mechanical Details
A.4.3.1
Spark Ignition at Pilot Tip
Generally, these systems require that an electrode capable of a capacitive high-energy or high-voltage discharge
be located close to the pilot-tip discharge. The electrode may be routed down the interior of the pilot or along the
outside of the pilot. It is necessary that electrode supports and/or penetrations into the pilot be constructed such
that they electrically isolate the electrode from the pilot.
In some cases, the electrode in this location serves the dual role of igniter and flame-ionization detector. See A.5
for more on flame ionization for pilot-flame detection.
The distance between the electrode and a high-voltage power supply is often limited to approximately 7.6 m
(25 ft). This limitation requires that the power supply be mounted on the flare stack. This distance is great enough
that the power supply is usually not subject to damage from the flame, but small enough that the power supply
cannot be accessed while the flare is in service.
59
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
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Key
1
2
ignition transformer with push button
sparkplug
3
4
to pilot
sight glass
5
6
mixing chamber
flow-control valves
a
Gas supply.
Air supply.
b
Figure A.13 — Flame-front Generator Panel Arrangement
60
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
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Key
1
2
3
4
5
6
a
b
flare-burner
relief-gas inlet
pilot assembly
pilot-gas line
drain
flame-front generator panel
gas supply
air supply
Figure A.14 — Flare/Pilot/Flame-front Generator Panel Arrangement
Low-voltage, high-energy igniters utilize a solid-state sparkplug that produces a spark, generally of 1 J or greater
energy. The high energy is produced from a capacitor-discharge system. These systems use a semiconductor
sparkplug from the aircraft industry in place of an air-gap spark device. The semiconductor acts as an insulator
that breaks down at a voltage generally less than 600 V and produces a high-energy spark. This voltage is
significantly less than the several thousand volts of a standard air-gap, high-voltage spark igniter. Low-voltage,
high-energy igniters can be located more than 914 m (1000 ft) from the energy source.
These igniters offer several advantages over traditional spark igniters. The lower voltage means that the wiring
between the exciter and spark device can be 600 V wiring suitable for anticipated temperature. The igniter can be
located on the order of a hundred meters (feet) away from the electronics package. This feature allows the
mounting of the electronics package for the igniter at grade, where it can be maintained and can be protected
from the radiant heat of the flare. In addition, the semiconductor surface is not as likely to foul as a traditional air-
61
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publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
gap spark system. The lower voltage is not as susceptible to grounding through insulators. The system is less
affected by rain and or carbon buildup. The system is less dependent on ceramic insulators, which are prone to
cracking in a flare application.
FO
The high energy of the igniter successfully ignites a wider range of air-fuel mixtures than a traditional spark igniter.
The igniter can be located inside the pilot’s air-fuel gas stream and away from the normal pilot tip and its flame.
The ignition of the pilot is almost instantaneous, making it well suited for automatic ignition systems. It is well
suited for wet gas or harsh environments where a greater energy is required to provide ignition.
Spark Ignition of a Portion of the Pilot Gas/Air Mixture Prior to the Pilot Tip
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A.4.3.2
These systems generally require that an electrode capable of a high-energy capacitive discharge be located in the
piping upstream of the flare-burner or in a bypass line between the piping and the flare-burner discharge. As with
the spark ignition system described above, it is necessary that electrode supports and/or penetrations into the
pilot be constructed such that they electrically isolate the electrode from the pilot.
The electrode in this system is not located in close proximity to the flame. This lack of continuous flame exposure
is often claimed to extend service life. The lack of flame exposure precludes use of the electrode for flame
detection.
A.4.3.3
Compressed-air Flame-front Generator
Compressed-air flame-front generator systems are usually built with a control panel. The control panel includes
the valves and orifices as well as the mixing tee, spark generator and sight glass. The spark generator is typically
either a sparkplug or a piezoelectric igniter. It is generally more convenient for cleaning and parts replacement if
the piping at the control panel is threaded. Either orifice unions or machined orifices may be used. The fuel- and
air-pressure gauges should either be liquid filled or installed with a snubber to prevent damage to the gauge due
to pressure pulses. All valves downstream of the point where the fuel and air are mixed should be full port. The
sight port at the spark generator should be designed for at least the same pressure as the piping. It is important to
design the system to prevent back-flow of one utility system into another.
The piping downstream of the control panel is usually constructed from welded DN 25 (NPS 1), schedule 40
carbon steel pipe. However, stainless steel pipe and/or threaded fittings can be used. Larger pipe sizes are more
likely to have the flame front transition to a detonation. Smaller pipe sizes are more likely to have the flame front
quenched before it reaches the pilot. Piping with equivalent lengths in excess of 1.6 km (1 mile) has been
successfully employed in flame-front generator service.
In order for a flame front to propagate down the flame-front generator line, it is necessary that the line be dry.
Consequently, it is essential that all the flame-front generator lines be sloped to drains that can be accessed,
opened and drained prior to use. In cold environments, the flame-front lines are often heat-traced and insulated to
prevent plugging due to freezing.
One flame-front generator can be used to light multiple pilots. This can be done one of two ways. The flame-front
generator can be connected to a manifold of valved lines, each of which ignites a single pilot. In this case, each
pilot is ignited individually. A flame-front generator can also be designed to light all pilots simultaneously with a
single branching flame-front line. If this is done, it is important to ensure that the flow to each line is balanced
adequately to ignite all pilots, regardless of the wind condition. The design with a single line that branches and
lights all pilots simultaneously has the disadvantage that the pulse from the flame-front generator can fail to light
all pilots. Then, a re-ignition attempt is required to again light all pilots rather than the single failed pilot. The flamefront generator pulse during this re-ignition attempt can possibly extinguish a working pilot.
A.4.3.4
Self-inspirating Flame-front Generator
A self-inspirating flame-front generator has a pressure-loss limitation because the fuel/air mixture is created with
an eductor. This pressure-loss limitation has a significant impact on the mechanical design. First, branching lines
and valve manifolds are generally not feasible; consequently, each self-inspirating flame-front generator is
dedicated to a single pilot. Second, the piping is usually limited to 90 m (300 ft) in length with very few turns.
Because of the pressure-drop limitations, the eductor and spark generator are usually mounted vertically near the
base of the stack. Assuming there are no horizontal sections and the eductor is open to the atmosphere, no
drains are required. In cold environments, the flame-front lines are often heat-traced and insulated to prevent
plugging due to freezing.
62
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
A.4.4 Operation
A.4.4.1
General
FO
During start-up, the operator should ensure that the flare system is free of oxygen prior to igniting the pilots.
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The flare line downstream of the main header blind should be purged with inert gas to reduce the oxygen levels to
safe proportions. The header should be purged with at least 10 times the free volume of the header with a noncondensable, inert gas. As a result of this purge, a maximum oxygen concentration of less than 6 % volume
fraction is recommended, unless process conditions indicate a more conservative level should be reached. After
the pilots are ignited, a hydrocarbon gas purge can be used.
A.4.4.2
Direct Spark Igniter
Systems that involve spark ignition at the pilot are either controlled by a central sequencing system, such as a
programmable logic controller (PLC), or have a push button on a control panel located at grade. Operation of
these systems is summarized as follows.
a)
b)
c)
Turn on the fuel to the pilot that is being ignited and set it at the manufacturer’s recommended pressure.
Allow adequate time for the fuel to reach the pilot tip.
Press and release the button to initiate the spark.
Monitor the flame-detection system for confirmation of pilot ignition.
If the pilot flame is not detected, repeat steps a) and b). If a pilot flame is not detected after several attempts, refer
to the troubleshooting guide in A.4.6.
A.4.4.3
Compressed-air Flame-front Generators
These ignition systems are the most complex to operate. Operation of these systems is summarized as follows.
a) Drain condensate from fuel and compressed-air supply lines to avoid entraining condensate into the flamefront generator.
b) Confirm that the fuel inlet valve is closed. Confirm that at least one pilot ignition line is open.
c) Open the air inlet and flush the flame-front generator line(s) with air.
d) Close the air inlet.
e) Open all condensate drains in the flame-front generator line(s). Clean the drains as necessary to ensure that
all liquid is removed. Close the drains.
f)
If the valves connect the flame-front generator to several separate pilots, open the valve to the pilot being
ignited and close all others.
g) While observing the sight glass, test the spark generator to confirm that a spark is generated. Alternatively, try
to detect radio frequency (RF) interference from a spark on an open channel of a radio, but this is directionally
less reliable than sight observation.
h) Turn on the fuel to the pilot that is being ignited; set it at the manufacturer’s recommended pressure. Allow
adequate time for the fuel to reach the pilot tip.
i)
Turn on the air to the flame-front generator and set it at the manufacturer’s recommended pressure.
j)
Turn on the fuel to the flame-front generator and set it at the manufacturer’s recommended pressure. Allow
adequate time to develop a sufficient quantity of pre-mixed fuel and air in the ignition line.
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publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
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k) Press and release the button to initiate the spark. Observe the sight glass and/or the pressure gauges at the
time of the spark. If the mixture ignites, a flame should be visible in the sight port and a pulse should be seen
in the pressure gauges. Do not hold down the button that initiates the spark. The intent is to ignite the mixture
once and then to allow the flame front to travel down the pipe to the pilot. Holding the button down does not
continue to generate a flame front and can stabilize a flame at the spark generator. A stabilized flame at the
spark generator quickly overheats the equipment in the vicinity of the spark generator. If such a stabilized
flame occurs, immediately close the fuel block valve, maintain airflow and allow equipment to cool. Restart the
ignition sequence from step j).
l)
If no flame is observed in the sight glass and no pulse seen in the pressure gauges when the spark is
generated, refer to the operations manual or contact the manufacturer. In most cases, an incorrect fuel/air
mixture is the cause and this typically requires field adjustment. See the troubleshooting guide in A.4.6.
m) Once ignition is observed at the control panel, wait for the flame front to reach the pilot. Allow an appropriate
amount of time for the flame front to reach the pilot. About 1 s to 3 s for each 30 m (100 ft) of piping is typical.
Consult the operations manual or manufacturer for the system specifics. Monitor the flame-detection system
for confirmation of pilot ignition.
n) When awaiting a successfully detected pilot ignition, some delay can occur with thermocouple detectors. Do
not quickly repeat FFG ignition attempts.
o) If the pilot flame is not detected, repeat steps k), l), m) and n). If a pilot flame is not detected after several
attempts, see the troubleshooting guide in A.4.6.
p) Once the pilot is ignited, flush the line with air only to remove corrosive combustion products. After the line is
flushed, close the valve leading to the ignited pilot and open the valve to the next pilot being ignited. Repeat
steps j), k), l), m) and n) for all pilots being ignited.
q) Once all pilots are lit, shut off the fuel to the flame-front generator, purge with air for several minutes to
remove all fuel and combustion products, then shut off the air.
A.4.4.4
Self-inspirating Flame-front Generator
Operation of this ignition system is generally simpler than that of the compressed-air type. The differences
between the former and the latter type of system are summarized below.
a) There is no compressed air. If mixture adjustment is required, it should be accomplished through fuel orifice
changes or eductor adjustments.
b) The system is not enclosed. If the flame-front line is plugged, a combustible mixture can back-flow out of the
mixer. If this is the case, do not initiate the spark. Never look into the mixer during ignition.
c) These systems seldom service more than one pilot; hence, there are rarely valves downstream of the mixer.
d) These systems are usually installed vertically with very little horizontal piping. Consequently, these systems
usually have no drains.
e) When the fuel supply is stopped, flashback can occur in the mixture tube. The user should decide whether
special measures are required to mitigate this flashback.
A.4.4.5
Operator Training
A plan for periodic training of operators in the use of pilot ignition equipment should be implemented. The training
should prepare the operator to properly operate the ignition equipment under adverse or urgent conditions and
can also serve as a periodic check of the functionality of the ignition system.
64
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publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
A.4.5 Maintenance
FO
Routine maintenance of systems that provide a spark at the pilot is limited due to their location. Generally, these
systems can be accessed only when the flare is out of service. When the flare is taken out of service, these
systems should be cleaned, inspected and, if necessary, replaced.
Routine maintenance of flame-front generator systems is focused on keeping the system dry and clear as well as
maintaining the valves, gauges and spark generator.
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The system should be installed with drain(s) at all low spots. The drains should be left closed when not in use, but
should be opened and cleaned regularly. The reason for leaving the drains closed when not in use is twofold.
a) If the drains are left open, exhaust gas from the pilot can circulate down through the flame-front generator,
promoting water condensation, corrosion and accumulation of moisture.
b) If the drains are left open and accidentally not closed prior to use, a vapor-cloud explosion can be generated
in the vicinity of the drain.
The pilot ignition system should be treated as an important safety-control system and should be inspected and
maintained on the schedule the plant has established for such safety systems. The fuel and air orifices as well as
the valves can occasionally require cleaning. The pressure gauges can occasionally require replacement or
recalibration. The spark generator can also require some routine service such as adjusting the spark gap. Periodic
functional checkout should be a part of the normal maintenance procedure and can be combined with an ongoing
training regimen for the operators.
A.6
Troubleshooting
Determination of pilot flame failure is covered in A.5. If a pilot is known to have failed and will not re-ignite, it is
important to understand first whether the problem is the pilot or the ignition system. Pilot problems can result from
the wrong pilot fuel, no pilot fuel, improper fuel-air mixture or loss of stability. The causes of these problems are
discussed in A.3.6. Ignition-system troubleshooting should be performed first because it can be done, to a large
extent, without requiring a plant shutdown. See Table A.3 for ignition-system troubleshooting.
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publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
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Table A.3 — Troubleshooting of Ignition Systems
Problem
Possible Cause
Corrective Action
Pilots with spark ignition at pilot tip
FO
Failed electrode: This can occur due to
extended flame exposure or exposure
to corrosive gases.
Replace electrode.
No ignition.
Liquid accumulation: Depending on
location, this might or might not be
possible. If the igniter is located
somewhere where liquids can collect,
this can isolate the spark from the
gas/air mixture.
Check piping arrangement to remove
low spots.
Check knockout drum operation.
No ignition.
Failed power supply.
Replace power supply.
No ignition.
Short: This can result from a failure of
the cable between the electrode and
the power supply or a failure of the
insulation between the electrode and
pilot.
Replace cable.
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No ignition.
Pilots with spark ignition of a portion of the pilot gas/air mixture prior to the pilot tip
No ignition.
Failed electrode: See above.
Liquid accumulation: See above.
See above.
Failed power supply: See above.
Short: See above.
No ignition.
Improper pilot fuel: In this system, the
spark lights a portion of the fuel-air
mixture supplied to the pilot tip.
Improper pilot fuel can cause flashback
or stabilization of a flame upstream of
the pilot tip.
Return to design fuel gas or modify
pilot to match the new fuel
composition. Pilot modifications can
include:
a)
b)
replacement of the pilot orifice,
adjustment of the air door (if any),
c)
replacement of the pilot entirely.
Pilots with a compressed-air flame-front generator
Failure to spark.
Failed spark generator, e.g. transformer Replace failed component.
Faulty ignition lead wire.
Damage to sparkplug.
Fouling or improper sparkplug gap are
possible causes of spark failure.
No fuel to flame-front generator.
Valves being closed or the fuelmetering orifice being plugged can
cause this.
Check valve position and/or orifice
cleanliness.
No air to flame-front generator.
Valves being closed or the air-metering Check valve position and/or orifice
orifice being plugged can cause this.
cleanliness.
66
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
Table A.3 — Troubleshooting of Ignition Systems (continued)
Problem
No flame present.
Possible Cause
Return to design fuel gas.
Restore original pressure settings.
Air pressure to flame-front generator.
Replace FFG fuel orifice to match the
Improper fuel characteristics can cause new fuel composition.
no flame or a detonation.
Refer to manufacturer’s instructions.
Improper fuel/air mixture: An improper
mixture does not support a flame front.
An improper mixture can result from
incorrect fuel or air pressure setting,
incorrect fuel or air orifice sizing or
improper fuel composition.
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Fuel composition and pressure to
flame-front generator.
Corrective Action
No flame present.
Plugged piping to flare: Ice formation
and debris are two examples.
Pressurizing the air supply only while
simultaneously observing the air and
fuel pressure gauges can identify
plugging.
High-pressure blowing to remove
debris.
Inject de-icing chemicals to melt ice
plug.
No flame present.
Moisture in piping to flare. This is one
of the most common problems in flamefront generators. A small amount of
moisture can quench the flame front. A
symptom of this problem is a seemingly
strong ignition, but no evidence of a
flame front reaching the pilot.
Purge flame-front generator and
ignition pipe with dry air prior to
attempting ignition.
Drain any low points in ignition piping.
No flame present.
Drain open in piping to flare. In an effort Check drain valve position or reinstall
to eliminate moisture in the piping,
drain plugs.
drain valves or plugs have been
accidentally left open. This can result in
the same symptoms observed with
moisture, but is far more dangerous as
combustible gas and/or a flame front
can be discharged at an unexpected
location.
No flame after several attempts.
Insufficient time for a flammable
mixture to fill the FFG line between the
FFG control panel and pilot.
Confirm the required time to fill the
FFG line with a flammable mixture.
No ignition.
Insufficient fuel gas or air to pilots.
Set appropriate regulators to the
proper pressure per manufacturer’s
recommendations.
No ignition.
Plugged fuel gas or pressured air
supply lines.
Check strainers/filters for blockage,
clean strainers or replace filters.
Pilots with a self-inspirating flame-front generator
No ignition or flame.
Failed spark generator: See above.
No fuel to flame-front generator:
See above.
Improper fuel to flame-front generator:
See above.
Improper fuel/air mixture: See above.
Plugged piping to flare: See above.
Moisture in piping to flare: See above.
See above.
67
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
A.5 Flame-detection Equipment
A.5.1 Purpose
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The flame-detection system confirms that the pilots are ignited. This is often confused with simple confirmation
that a flame exists. While these two statements are usually synonymous, there is an important difference. If the
pilots are ignited and a volume of inert gas is released, the flare flame is extinguished only while the inert gas is
being discharged. If the pilots are not ignited, but the flare is, and a volume of inert gas is released, the flare flame
remains extinguished after the inert-gas release and until a pilot can be ignited. If the pilots are not ignited
because they have failed, the flare can remain unlit for an extended period of time. Consequently, it is important to
confirm both the presence of a flame and also the presence of a pilot flame.
A.5.2 General Description
A.5.2.1
General
A pilot-flame detection system utilizes the heat, ionized gas, light or sound generated by a pilot flame to verify that
a pilot is burning. An example of the use of each of these energy sources for flame detection is described in
A.5.2.2 to A.5.2.5 respectively.
A.5.2.2
Thermocouples
The most common flare pilot-flame detector is a thermocouple. Thermocouples have the advantage that they
detect the pilot flame only and are not directly exposed to the flare flame. Relative to other types of detectors,
thermocouples have the disadvantage of a relatively slow response time and an often limited service life. The
limited service life is a frequent problem, since in most cases, the thermocouples cannot be replaced while the
flare is in operation, unless a retractable type is used.
A.5.2.3
Flame Ionization
A flame generates ionized gases within the flame envelope. Flame-ionization detectors function based on a
change of resistance between two electrodes. Flame-ionization detectors have the advantage that they respond
rapidly and that they detect the pilot flame only. The disadvantage of flame-ionization detectors is that it is
necessary that they be exposed directly to the flame. Similar to a thermocouple, the flame-ionization detector is
mounted on the pilot; hence, if it fails, it cannot be serviced while the flare is in operation.
A.5.2.4
Optical Systems
There are two types of optical sensors: ultraviolet (UV) and infrared (IR). Both types have the advantage of being
located at grade where they can be serviced while the flare is in operation. Both types can indicate false flame
failures caused by obscuration due to clouds or precipitation. The most significant disadvantage concerning
existing designs of these detectors is their inability to detect the pilot flame separately from the main flame. Given
this disadvantage, these detectors are not recommended for use as the sole means of pilot-flame detection.
A.5.2.5
Acoustic Systems
A flare pilot flame generates a characteristic sound when it is burning. If the pilot flame is extinguished, the sound
changes. The pilot sound is conveyed to grade where an acoustic pilot-flame detector monitors the pilot sound
and signals the pilot operating condition. The acoustic detector differentiates the monitored sound from other
sounds in the vicinity of the pilot. An acoustic pilot-flame detector can be installed and serviced while the flare is in
operation.
68
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
A.5.3 Mechanical Details
A.5.3.1
Thermocouples
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Typically, Type K thermocouples in type 310 SS or Inconel7) sheaths are used. The sheathing should extend
throughout the zone that can experience flame impingement, typically at least 1.8 m (6 ft) or 125 % of the actual
flare-burner diameter. In order to improve the service life, thermocouples are sometimes located in a thermowell
that is incorporated into the pilot design. In addition, some designs today are available with multiple
thermocouples, such that if one fails, a second can be put into service. There are also systems that are now
offered with retractable thermocouples that can be replaced while the flare is in service.
Retractable thermocouple systems usually consist of a smoothbore conduit with a limited number of bends that
extends from grade to the pilot. A coiled, flexible thermocouple is inserted from grade though the conduit. These
systems improve reliability when properly installed by allowing thermocouple replacement while the flare is in
service.
A.5.3.2
Flame Ionization
Two electrodes are required for a flame-ionization system. In practice, the pilot tip is usually used as the ground
electrode. A conductor of much smaller area is positioned in the flame envelope and functions as the other
electrode. An alternating voltage is applied across the electrodes; direct current flow is monitored as a means of
flame detection.
A.5.4 Optical Systems
Optical systems are mounted at grade and it is necessary that they be directed at the flare. All optical systems
have some means of aiming the detector, either sighting directly through the detector optics or through a scope
mounted on the detector. The detectors should be mounted on an easily accessible, vibration-free platform that is
located one to three stack heights from the flare-burner. Most optical systems have a maximum range of 150 m to
300 m (500 ft to 1000 ft), which is adequate for most flares. Infrared detectors should be located such that at no
time during the day does the detector face directly into sunlight. In the northern hemisphere, this means that the
detector should be mounted on the south side of the stack. Sometimes two sensors are used, and installed in
opposite directions, to improve the reliability of operation.
The electronics associated with optical detectors are usually mounted close to the detector. Local status lights
and alarm strobes are available from some manufacturers.
A.5.5 Acoustic Systems
An acoustic pilot-detection system consists of a sensor unit and a signal-processing unit. Wiring interconnects the
units. The pilot sound is normally conveyed to grade via the piping connecting the pilot to the flame-front
generator. The sensor unit is attached to the pilot’s flame-front generator pipe near the base of the flare stack.
The signal-processing unit may be placed of up to 365 m (1200 ft) distant from the sensor unit.
A.5.6 Operation
A.5.6.1
Thermocouples
With thermocouple systems, temperature greater than the minimum set by the pilot manufacturer indicates the
presence of a pilot flame. Some manufacturers have accelerated flame detection by monitoring the rate of
temperature change as well as its magnitude.
®
7) INCONEL is a registered trade name on INCO family of companies. This is an example of a suitable product available
commercially. This information is given for the convenience of users of this standard and does not constitute an endorsement
by API of this product.
69
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
A.5.6.2
Flame Ionization
The presence of a flame between the electrodes creates a current flow between electrodes. The difference in
electrode areas is such that the resulting current is rectified (flows preferentially in one direction). A short in the
system creates current flow, but it is not rectified. Hence, the rectified current is indicative of a flame.
FO
A.5.6.3
Optical Systems
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As mentioned above, both IR and UV systems are available. Due to the significant background levels of IR
radiation in the atmosphere, an infrared detector appropriate for detecting flare or pilot flame uses more than
simply the magnitude of the IR radiation within a certain or single waveband. Infrared detectors make use of the
fact that flames emit IR radiation at relatively discrete frequencies. By observing the IR radiation in two bands, one
characteristic of flame emission and one not a ceratin ratio of the two signals is indicative of a flame.
There is substantially less background UV radiation, as compared to IR radiatrion, and flames emit UV at very
discrete wavelengths. Consequently, UV systems simply monitor the magnitude of UV radiation in a particular
waveband (usually that associated with emissions from OH radicals). UV is absorbed by regular glass so the
optics in a UV system are generally constructed from quartz. The sun also emits UV radiation. The positioning of
UV-based detectors should avoid interference from sun-generated UV.
A.5.6.4
Acoustic Systems
With acoustic systems, pilot sound is continuously monitored by the system. A change in pilot status is detected
and indicated via the local status lights and the dry contacts. Factory signal-processor settings can be field
adjusted to account for site conditions such as the complexity of the piping from the pilot to the sensor.
A.5.7 Maintenance
A.5.7.1
Thermocouples
Unless a retractable thermocouple system is installed, there is very little maintenance that can be performed on a
thermocouple, other than monitoring its performance to ensure that it has not failed. If it fails and more than one
thermocouple is installed, the spare thermocouple can be put into service. If it fails and is retractable, it can be
replaced.
A.5.7.2
Flame Ionization
There is virtually no maintenance that can be performed on a flame-ionization system, unless the pilot is taken out
of service. The only maintenance possible during operation of the flare is on the controls “at grade”.
A.5.7.3
Optical Systems
The sighting of the optical systems should be checked regularly to ensure that the flare-burner remains in the field
of view. The optics should be cleaned periodically.
A.5.7.4
Acoustic Systems
The sensor and signal-processing units of acoustic systems are located at grade and are accessible during flare
operation. The drains in the sound-conveying piping should be checked on a regular schedule.
A.5.8 Troubleshooting
See Table A.4.
70
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publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
Table A.4 — Troubleshooting of Flame-detection Systems
Problem
FO
Pilot detector is suspected of being in
error.
Possible Cause
Corrective Action
Pilot and its ignition system are
Inspect with binoculars or telescope.
believed to be functioning correctly,
Inspect at night.
check corrective-action steps to confirm
Use the FFG to supply additional fuel
pilot ignition.
to make the pilot flame more visible.
Perform a functional check based on
the manufacturer’s instructions.
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Pilot detection system is determined to Check electrical supply and fuses.
be showing a false loss of flame when
one is present or false confirmation of
flame when it is absent.
Thermocouple failure.
Check for open circuit.
Thermocouple sensing flare flame
rather than pilot flame.
Check wind direction and flame
position relative to pilot in question.
Flame-ionization electrode failure.
Check for open circuit.
Flame-ionization electrode shorting.
The signal processor should recognize
this as an unrectified signal. Check
signal processor.
Flame-ionization electrodes sensing
flare flame rather than pilot flame.
Check wind direction and flame
position relative to pilot in question.
Improperly aimed optical system.
Check view angle.
Optical system obscured by clouds or
fog.
Check line of sight.
Optical system obscured by dirty optics Inspect optics and clean as required.
Pilot detector is suspected of being in
error.
Optical system sensing flare flame
rather than pilot flame.
This is a limitation of the optical
sensing devices. No corrective action
is currently possible.
The acoustic-system sound-conveying
path may be blocked.
Check drains. Check for displaced or
damaged piping. Confirm that the path
is clear.
Suspected false confirmation of flame
by acoustic system.
Check first by disconnecting the
sensor unit from the sound-conveying
piping and then by covering the sensor
inlet. The system should then indicate
the pilot is out.
Interconnecting wiring and its terminals Replace or repair wiring.
may be compromised or faulty.
Control units may be faulty or be
suffering from the effects of an
aggressive environment.
Replace components.
71
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
A.6 Verification Test
A.6.1 General
FO
To ensure the pilots, ignitors and detectors perform under the severe weather-condition requirements of this
Standard, a production unit of each combination of pilot, ignitor and detection system should be tested to verify
the ability to meet the required performance. The following is one method of such a test protocol.
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A.6.2 Setup
The test should be performed at a test facility equipped with the following:
 Stack, DN 450 (NPS 18) or larger, for mounting the pilots can ensure that the effects of eddy currents around
the stack are simulated.
 Air blowers that can produce the required wind speed. The volume and discharge of the air should extend
beyond the entire diameter of the stack and all of the upper portions of the pilot; this ensures that the
simulated wind speed is consistent in the area of the pilot tip. If the pilot has an air inspirator, there should
also be a similar air discharge to produce wind over and around the inspirator.
 The air discharge should be equipped with water injection to produce the required amount of rain.
 Suitable, calibrated instrumentation should be used to verify the wind speed and the amount of rain.
 The stack should be equipped appropriately so that the pilot can be mounted in upwind, crosswind and
downwind positions to ensure that the performance is measured under different wind directions.
A.6.3 Testing
The performance test should include testing the combination of pilot, ignitor and detector as appropriate for all
three positions around the test stack. Record the data for each component at each position as follows.
a)
Pilot: Record the fuel pressure, fuel composition, fuel flow rate and net heat release used for the test.
Record the maximum wind speed without rain at which the pilot remains stable. The pilot flame should be
proven to be stable for at least 3 minutes after the airflow is established. For each position, also test and
record the maximum wind speed with rain at which the pilot remains stable. Record the amount of rain for
each test.
b)
Ignitors: Record the maximum wind speed at which the pilot ignites three consecutive times. Provide typical
details on the type of ignitor used. For FFG, include air and fuel flow rate, pressure and composition/heat
release of the fuel. State the model of pilot on which the test was performed. For each position, also test as
per the above with a recorded amount of rain. For electronic systems, record how long it takes to light the
pilot each time.
c)
Detectors: Record the maximum wind speed at which the detector can monitor the flame consistently three
consecutive times. Provide typical details on the type of detector used. For thermocouples, record the
temperature of the thermocouple without wind and the temperature after 3 minutes of stable operation with
the corresponding recorded wind speed. For all types of detectors, turn the pilot fuel gas off and record the
length of time required for the monitor to detect that the pilot is out. Do this with and without wind. Perform
the above with rain and record the corresponding amount.
A.7 Purge-gas Conservation Seals
4.7.1 Purpose
All flare systems are susceptible to flashback and explosion if not properly purged to keep air (oxygen) from
entering the flare stack downward through the flare-burner. To prevent air from entering the system during normal
operation, a continuous purge is required. A purge-conservation device may be installed in (or immediately below)
the flare-burner in order to reduce the purge-gas consumption. See API Standard 521 for purge rate guidance.
72
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publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
4.7.2 General Description
FO
Purge-conservation devices are designed as stationary mechanical components. They reduce the use of purge
gas while preventing some (but not all) air from entering downward into the flare stack. Several names are used to
describe these seals, but some generic names are velocity seals, venturi seals, buoyancy seals and diffusion
seals.
4.7.3 Mechanical Details
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Refer to API Standard 521 for examples and requirements for these seals.
A.7.4 Maintenance
Buoyancy or diffusion-type purge seals have several flow reversals. These act to separate the liquids from the
flare gas. These liquids can accumulate in the bottom of the seal and cause several concerns, including blockage
of the seal, corrosion and additional structural loads. Seals are equipped with a drain connection and separate
drain line to allow removal of these liquids and elimination of these problems.
The drain should be designed in a manner to mitigate plugging and equipped with an inspection opening to allow
cleaning of the drain during shutdowns. It is important that the drain be sealed to prevent the entry of air. This is
often accomplished by the use of a liquid-filled loop seal sized for at least 1.75 times the sum of the calculated
th
seal and flare-burner pressure drop at the maximum flow rate. See API Standard 521, 6 ed., 5.8.10.2.
This seal leg should have a level indication and alarm. The drain line should allow for routine back-blowing with a
purge gas containing no oxygen. A pressure gauge should be used to ensure the line is open and free.
Velocity or venturi-type seals do not normally require maintenance except for inspection and cleanout at
shutdowns. To avoid liquid accumulation, it is necessary that velocity seals incorporate drain holes at their base.
A.7.5 Troubleshooting
See Table A.5.
73
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publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
Table A.5 — Troubleshooting of Purge-gas Conservation Seals — Buoyancy Type
Problem
FO
Burning through seal (hole in top or
sidewall); hole in the side of the flareburner.
Possible Cause
Internal burning.
Corrective Action
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Check drain loop seal. Restore liquid
level if necessary.
Temporary: switch to nitrogen or inert
purge. Repair or replace as soon as
possible. Divert to backup or rental
flare.
Corrosion from steam condensate from Temporary: switch to nitrogen or inert
smokeless flare.
purge. Repair or replace as soon as
possible. Divert to backup or rental
flare.
Excessive pressure drop.
Purge-gas conservation device is
plugged.
Clean drain or replace device.
Freezing due to steam condensate
from smokeless flare or rain
accumulation.
Blow drain (if equipped) with highpressure hot glycol or alcohol mixture.
Check that the heat tracing is
operational, if installed. Divert to
backup or rental flare.
Carbon buildup due to internal burning. Blow drain (if equipped) with highpressure gas to clear. Use pipecleaning service, if possible. Repair or
replace as soon as possible.
Divert to backup or rental flare.
Refractory cracking and spalling
causing plugging of the drain and the
bottom of seal.
Blow drain (if equipped) with highpressure gas to clear. Repair or
replace as soon as possible. Divert to
backup or rental flare.
Excessive sway at the top of the flare.
Purge-gas conservation device is
plugged and full of liquid or debris.
Clean drain.
Leakage from the base of the seal.
Corrosion due to plugged or fouled
drain.
Clean drain with high-pressure steam
or gas. Temporary: switch to nitrogen
or inert purge. Repair or replace as
soon as possible. Divert to backup or
rental flare.
Liquid carryover (burning rain).
Hydrocarbon condensate buildup and
accumulation, carried out through the
seal and flare-burner by large gas
flows; excessive liquid buildup is
normally also seen as excessive
pressure drop. Small amounts of HC
liquid do not cause a noticeable
pressure drop, but are easily swept up
and out of the flare by large flaring
rates.
Check knockout-drum liquid level.
Drain liquid if level is too high.
Purge-gas conservation device is too
small or is partially plugged by carbon,
ice or refractory.
Clean drain or replace with a device of
larger diameter. See above for
recommendations regarding excessive
pressure drop.
Noise.
Clean drain. Blow drain with highpressure steam or hot nitrogen. Start
blow-down procedure slowly and
watch for burning rain; gradually
increase blow rate. Check that heat
tracing is operational, if installed.
Divert to backup or rental flare.
74
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publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
A.8 Knockout Drums and Liquid Seals
A.8.1 Knockout Drum
FO
A.8.1.1
A flare knockout drum separates liquid from gas in a flare system and holds a specified amount of
liquid that can be relieved during an emergency situation. See API Standard 521 for the amount and size of liquid
droplets that can be handled smokelessly by the flare-burner.
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A.8.1.2
There are three basic types of knockout-drum designs that can be incorporated into a flare system: a
horizontal settling drum, a vertical settling drum and a vertical centrifugal separator.
A.8.1.3
Knockout drums are typically located on the main flare line upstream of the flare stack or any liquid
seal. When there are particular pieces of equipment or process units within a plant that provide major sources of
liquid to the flare, it is desirable to have knockout drums inside the battery limits for these sources. This reduces
the sizing requirements for the main flare knockout drum as well as facilitating product recovery.
A.8.1.4
API Standard 521 describes flare knockout-drum orientation, design criteria, instrumentation,
mechanical details, operations, maintenance and troubleshooting.
A.8.2 Liquid Seals
A.8.2.1
Flare liquid seals are designed to:
a) prevent any flashback originating from the flare-burner from propagating back through the flare system,
b) maintain a slight positive system pressure to ensure that there is no air leakage into the flare system and to
permit the use of a flare-gas recovery system,
c) provide a method of flare staging between a smaller capacity smokeless flare and full-size emergency flare,
d) prevent ingress of air into the flare system during sudden temperature changes such as that following a major
release of flare gas or steaming to flare.
A.8.2.2
Liquid seals are located after the main knockout drum and before the flare itself. Elevated flares can
be equipped with a separate seal drum or can incorporate the liquid seal into the base of the flare stack.
A.8.2.3
Liquid seals should include an internal wave attenuator. Typically, such devices are located around
the inlet pipe.
A.8.2.4
The liquid seal and purge system should be designed to prevent the seal from being broken as a
result of the vacuum formed in the flare header following a major release of flare gas or steaming, as specified on
General flare data sheet 5 in Annex F. See also the data sheet instructions for General flare data sheet 5 in
Annex E.
A.8.2.5
Liquid seals may be used in services below 0 °C (32 °F) if the effect of the cold fluid on the seal liquid
is taken into account. Water seals are not recommended where there is a risk of obstructing the flare system due
to an ice plug. Alternative sealing fluids, such as stove oil or a glycol/water mixture, may be considered. If a
flammable seal fluid is used, consider that liquid carryover from a liquid seal can occur during a major flaring event.
A.8.2.6
API Standard 521 provides additional information for liquid seals.
A.8.2.7 Whenever a liquid seal is not integral (coaxial) with the stack, proper considerations should be given to
the stress analysis of the connecting lines, with particular concern given to thermal stresses and to the possible
formation of liquid slugs during emergency discharges.
75
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publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
A.9 Blowers and Drivers
A.9.1 Purpose
FO
A blower (fan) is used to provide air and turbulence to the combustion process to make a flare smokeless. The
circumstances when this should be considered are addressed in A.2.5. Section A.9 describes some design and
practical considerations associated with such devices.
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A.9.2 General Description
In general, three types of blowers have been used for this duty:
 centrifugal: commonly backward-curved bladed fans used with electric-motor drives; internal combustion
engine or steam turbine drives have been used under special circumstances (see Figure A.15);
 vaneaxial axial-flow fans with fixed or adjustable pitched blades: commonly used with close-coupled electricmotor drives up to 260 kW (350 HP) (see Figure A.16);
 high-pressure blowers: commonly multi-stage centrifugal blowers with a wide variety of impeller configurations
used with discharge pressures, typically up to 103 kPa (15 psig) or more.
Blowers can be used singly, in multiples allowing stand-by or in multiples allowing added capacity (see A.9.3).
The drives can be single-, double- or variable-speed, and with or without other flow control devices.
A.9.3 Mechanical Details
The following mechanical details should be considered.
a) If multiple blowers are provided, attention should be paid to the potential of air from the active blower passing
through the inactive (idle) blower. Back-draft dampers or “anti-sail” pawls on the blower drive should be
provided to forestall problems. There are two principal effects of this air loss.
1) The escape of air through this route lowers smokeless capacity.
2) The passage of air causes the blades of the idle blower to rotate in the opposite direction to that of the
driven condition. Thus, when it is necessary to start the blower, it is necessary that the motor stop
spinning in the reverse direction. This extends the period of low-speed operation. It is common that
electric-motor starters can trip on thermal overload during this extended period of low inductance. In
extreme cases, it is possible to break the driveshaft under these conditions.
b) The blower selection, installation and operation are an essential part of the performance of the flare. It is
recommended that the flare manufacturer be given design and supply responsibility to reduce operational
problems.
c) Care should be taken when applying purchaser standards to this blower. The specifications for a blower for
smokeless burning with an air-assisted flare-burner might not conform to the specifications for blowers applied
to process plant requirements. These purchaser fan specifications typically reflect the use of much larger fans
under very different service conditions, e.g. as specified in API Standard 560. The blower on an air-assisted
flare is rated for a peak duty to generate a maximum smokeless-burning capacity. This blower rarely operates
at this high load condition. Normal air-assisted smokeless-flare blower operations should be at a reduced
operating condition. It is not typical to rate a smoke-suppression blower with the over-rating on airflow and/or
pressure that is typical for a process blower where the blower is fully loaded and critical to plant operations.
d) The driver for a blower for an air-assisted smokeless-flare operation differs from those applied to process
blowers. As noted above, it is not necessary that the horsepower rating reflect the over-rating for additional
airflow and/or pressure drop. The driver rating may also be engineered with regard to the typical load cycle of
a smoke-suppression blower where high flow rates and pressures are atypical of daily operating conditions. It
is necessary that the specifications for an electric driver reflect the flare operating-area requirements and the
type of blower applied. Typically, totally enclosed motors are applied to meet electrical-area classifications.
76
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
FO
e) For centrifugal blowers, the motor is totally enclosed and fan-cooled (TEFC). For direct-drive axial blowers, a
totally enclosed, air-over (TEAO) motor is typically applied. A TEAO motor is essentially a TEFC without the
cooling fan. The TEAO motor uses the blower airflow for cooling and is more efficient and can have a higher
power availability than does a TEFC motor. Normal electric-motor specifications can cause problems when
improperly applied, e.g. most sites require all motors to be TEFC, but the vaneaxial design in AMCA 801
arrangement 4 uses a TEAO motor design.
f)
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The blowers used for air-assisted smokeless flares can contribute to the plant noise levels. For overall noise
management, mufflers and noise enclosures are sometimes used. Consideration of local noise levels at the
blower should include the flare location and the worker exposure time to the blower noise. If the blower is
operating at a high rate, there is significant flaring and it can be atypical for service personnel to be at the
blower location and exposed to such blower noise for extended time periods. It is necessary that the impact of
the application of blower mufflers and enclosures on the blower performance and power requirements be
considered.
g) Winter operation will cause the motors to draw more current than summer conditions. Selection of winter
thermal overload settings and breaker settings needs to take into account this increase. In a TEAO
configuration, the additional cooling from blowing very cold air over the motor prevents the motor from
overheating.
h) Blower control options that provide enhanced flow control and substantial energy savings are available. These
options include inlet-vane dampers, controllable-pitch vaneaxial fans, multiple-blower combinations and
variable-speed drives.
i)
It is necessary that the attachment of the blower and/or its ducting to a flare stack consider the relative
movement and structural loads of the attachment. For instance, if a fixed blower foundation is used, it is
necessary that the ducting between the blower attachment and the flare stack take into consideration the flare
stack movement at the point of attachment.
Key
1
2
inlet collar
inlet
8
9
rim
blades
10 blast area
11 cutoff
5
6
impeller
backplate
12 outlet
13 outlet area
7
supports
3
4
housing
scroll
Figure A.15 — Centrifugal Fan
77
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
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Key
1
2
inlet cone
motor
3
4
outlet cone
fan
a
Airflow direction.
Figure A.16 — Vaneaxial Fan
A.9.4 Operations
Operation of a blower fitted to an air-assisted flare should take into consideration the following.
a) Over-aeration can cause the following problems:
1) flame blowout,
2) excessive noise (low frequency),
3) lower combustion or destruction efficiency,
4) shorter flare-burner life.
b) Most large motors have limits on the number of cold and hot starts per hour. The system should be designed
and operated to avoid excessive restarts.
c) The blower should be operating (at least at low airflow) at all times for cooling and when there is the potential
of a back-flow of flare gas (especially of high relative molecular mass) within the air riser. At least one blower
should be operating at all times in a multi-blower system.
A.9.5 Maintenance
Unlike many components of a flare system, blowers are generally in a position where they can be maintained.
Motor and blower vendors should provide maintenance schedules for their equipment showing recommended
frequency for actions, such as:
a) motor bearing and impeller lubrication as required;
b) free operation of back-draft dampers or anti-sail devices;
c) operation of flow-control devices, together with associated controls and actuators;
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publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
d) checking of motor-speed control devices and starters (as appropriate);
e) addressing out-of-balance or unusual vibration conditions;
f)
ensuring open passage to air entry (e.g. silencers, screens);
FO
g) replacement of any wearing parts, such as rotating unions, for controllable pitch fans.
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Blowers are normally located at the base of the flare. It is necessary that consideration be given to the degree of
radiation to which both equipment and personnel are exposed. Mitigation of such exposure can include taller flare
stack and radiation shielding.
A.9.6 Troubleshooting
Possible problems with blowers that are specific to their use in a flare system are ultimately related to the
production of unexpected smoke in a flare flame; such smoke is not necessarily caused by the blower system.
Table A.6 lists potential problems and possible causes that can generally be checked without shutting down the
flare system. The flare vendor and/or the blower vendor should be contacted to determine if there are any other
possible problems/causes that are application-specific.
Table A.6 — Troubleshooting of Blower Systems
Problem
Blower not moving.
Possible Cause
Corrective Action
Tripped overload or breaker.
Check for short circuits in power wiring
to blower motor. Reset overload or
breaker.
Incorrect power.
Check feed voltage and current draw
on each phase leg. Correct any wiring
problems revealed by this check.
Provide correct voltage to the blower.
Local blockage at blower inlet.
Inspect blower inlet. Remove any
debris or obstructions
Incorrect fan speed.
Confirm fan speed.
Incorrect blade pitch.
Stop blower. Record actual blade
settings on each fan blade. Compare
average blade setting to design
setting. Adjust as required to match
design setting.
Improper fan selection.
Measure fan outlet pressure. Compare
to fan selection basis. Adjust fan
blades (if possible) to obtain proper
airflow.
Incorrect airflow control.
Malfunction of flow control (e.g. inlet
vane damper, blade pitch or speed
control).
Verify proper operation of any flowcontrol systems associated with the
blower.
Reverse airflow.
Reversed power wiring.
Confirm proper wiring. Confirm correct
direction of rotation of fan by shutting
off power to the fan and observing
rotation as the fan slows down.
Blockage at blower outlet.
Confirm position of outlet damper.
Back-flow from another blower.
Confirm position of outlet damper.
Insufficient airflow.
Reverse airflow when blower should
be off.
79
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
A.10Blower Staging and Control Equipment
A.10.1 Purpose
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In an air-assisted flare system, the objective is to achieve satisfactory performance, i.e. smokeless operation over
the design range without undue noise or waste of power, and possibly over a wide range of flow and gas
composition. An auxiliary control system may be needed to determine what combination of blowers and operation
of controls is required to achieve such performance.
A.10.2 General Description
The equipment falls into three categories:
a) Detection: The condition of the flare system is continuously monitored and the system is operated in response
to the parameters of this monitoring. This detection can be as simple as monitoring the flare-gas pressure (by
pressure switch or transmitter). However, if the temperature or gas composition can change substantially,
such control can become more complex, possibly with flare and airflow measurements and/or smoke or
infrared detection with added modification from such controls. See A.11. Feedback control on blower output
can also possibly be required.
b) Logic: It is necessary to process the signal(s) from the detection phase to give appropriate control action. This
can be done by local dedicated control systems (e.g. PLCs) or the logic can be passed to a central control
system (e.g. DCS).
c) Flow control: The control system from b) is arranged to provide the appropriate signals to provide control. This
can be as simple as arranging for an increase in motor speed or the starting of another blower motor.
However, it may be necessary for the control to be as complicated as a sophisticated logic system that uses
all or some of the following techniques: stepwise speed control, continuous motor-speed control and/or
additional capacity addition, continuous flow control (by damper). In all control schemes, a readily available
manual override control should be fitted.
A.10.3 Mechanical Details
The selection of blower and blower control equipment for an air-assisted smokeless flare is a function of
 the smokeless burning requirements,
 the flare design and
 environmental parameters.
The quantity of forced-draft air required for smokeless burning is the most common design consideration. Often,
this is some fraction of the stoichiometric combustion-air requirement. The air quantity alone, though, does not
determine the smokeless-burning performance. The air velocity at the mixing point with relief gas is also an
important factor. This is the air energy at the flare-burner. The stoichiometric air fraction used for smokeless
burning can be significantly altered by the velocity employed at the flare-burner and by the flare-burner design.
Blower energy expended to deliver the air to the flare-burner does not effectively contribute to the smokeless
burning. Care is required in the design of the air-delivery piping/stack and connection of the blower(s) to the flare.
Obstructions in the air-delivery system, like flanges on an internal gas riser, can significantly contribute to blower
energy losses.
The blower airflow is designed for a maximum smokeless-burning rate with maximum design airflow. The flare
system should operate at reduced smokeless-burning capacities without excessive use of energy and without the
generation of an unstable or noisy flame. The number of blowers and the type of airflow control employed are
functions of the size, type and burning requirements of the flare system. On some systems, a single blower is
sufficient. Reduced smokeless flaring, on such a system, can be achieved with the use of a multiple-speed blower
motor, a variable-speed blower motor or the use of blower inlet or outlet dampers. The least operating power is
realized with the use of a multiple-speed or variable-speed motor. An inlet damper also reduces operating power.
Outlet dampers do not lower the operating horsepower.
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In the simplest control system, a multiple-speed blower motor is advanced to high speed or returned to low-speed
operation on a flow or pressure signal from the flare-relief header. It is recommended that the advancement to
high speed and the return to low speed incorporate some signal hysteresis. If a single switch point is used, the
blower can cycle between speeds excessively due to flow/pressure variations in the flare header. Attention should
be paid to the practicality of pressure detection if there is a wide flow range (the gas pressure varies directly with
the square root of the flow over the relevant range). Special care should be paid to the setting of the “deadband” to
avoid frequent blower stopping and starting at a particular range of flare flow.
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It is necessary that the air-assisted smokeless flare be designed to ensure that the blower-speed selections
supply sufficient energy to produce smokeless burning at the maximum rate while offering sufficiently low air
energy at reduced rates to burn stably at minimum flow rates.
If a variable-speed motor or inlet damper is used for proportional control of smoke-suppression airflow to relief-gas
flow, then reliable instrumentation to monitor the flow of flare vapors in the header is required. It is necessary that
this flow measurement consider the full operating range of the flare-system design. Air-assisted smokeless flares
are often designed for large relief rates where smoke is acceptable while offering smokeless burning at much
lower flow rates. For such systems, measuring low-speed, low-pressure flows in a flare header can be difficult.
Refer to A.11.
Continuously variable airflow control over the design range is possible by continuous motor-speed control
(e.g. steam turbine or internal combustion engine) or by a conventional damper control. To control such a system
requires suitable detection devices, e.g. a pressure transmitter, a flare-gas flow-metering control or smoke control
(e.g. infrared detector). Care should be taken to ensure that the airflow response is rapid enough to avoid
troublesome emissions of smoke during flow-change operations. It can be necessary to modify the control action
to reduce the effects of control-loop lag.
The use of multiple blowers adds an additional consideration for the air-assisted smokeless flare design. It is
important to ensure that the full operation of all blowers is engineered in detail from their start-up sequencing
through their maximum flow operation. Multiple blowers can be equipped with multiple-speed motors, variablespeed motors and/or dampers. The integration of these devices should include a consideration as to how airflow
is regulated from a minimum to a maximum. Any time an additional blower is required, it is necessary that its startup consider the operating condition in the flare air-delivery system. A second or third blower starts up against a
static pressure in the flare system. Such start-ups can cause blower surging and instability. Any blower surging
and airflow instability can be amplified by the combustion at the flare-burner to create an unstable, noisy flare
flame. Likewise, a blower engaged on a low-speed motor might not have sufficient static-pressure capability to
add airflow to an air-assisted flare that has an airflow already established by a blower operating on a high-speed
motor. The lower speed fan is discharging against a “dead-head” and the fan does not contribute to the common
flow until its discharge pressure is sufficiently high to cause flow. With two identical blowers, it is necessary that
both operate at the same speed to cause significantly more flow than one blower alone. The control scheme
should be arranged with this in mind.
It is necessary that the entire airflow system be engineered to ensure that proper airflow to hydrocarbon-gas relief
rates can be achieved over the full range of flaring conditions with any multiple-blower system. It is necessary that
this engineering consider the starting and stopping of blowers as well as the airflows that they produce. Largehorsepower motors can be started only a limited number of times per hour. It is necessary that the system design
address how to avoid excessive cycling of blower operations with transient flow conditions to the flare header.
Most important, the multiple-blower operations should address manual operator inputs to adjust airflow for
smokeless, low-noise operations.
A design consideration for multiple-blower systems is to have isolation dampers for any blower that is out of
service. This is to prevent airflow from discharging out of the idle blower. The opening and closing of these
isolation dampers needs to be considered in the design of the control logic of an air-assisted smokeless-flare
system with multiple blowers.
Control systems for multiple blowers can be by pressure/flow switch operation or can be by proportional control to
the flare relief flow.
Variable-speed drivers can include electric, steam or internal combustion power. A hybrid system that uses a
steam-driven blower motor can incorporate a steam assist for smokeless burning, using the exhaust steam from
the turbine drive.
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publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
The connection of an air-assisted blower to a flare stack should consider the loads due to blower operations and
the movements due to stack deflections.
A.10.4 Operations
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Operation of an air-assisted smokeless-flare system should start with an initial check-out of the blower operations.
Each blower should be checked for proper rotation and speed. The motor amperage should be confirmed for all
operating points with multiple blowers. It should be noted that many of the axial-type blowers used for air-assisted
flare applications have blade-pitch adjustments that can affect the air delivery. The blower-blade pitch should be
set to the maximum allowed by the motor or as otherwise recommended by the flare-equipment manufacturer.
The blower initial check-out should confirm the operation of damper, isolation or flow control. The blowers/motors
should be checked for vibration limits.
Blower controls should be checked for proper blower speed and multiple-blower sequencing. Operator indication
of operating blowers, blower speeds and damper positions should be provided. Maintenance items for the blowers,
motors and controls should be located such that they are accessible while the flare remains in service. Some
items exposed to radiant flare flame loads can require shielding.
Smokeless flaring operations should cover the range of flare relief-gas compositions and flow rates specified for
the flare operation. Operator intervention into blower operations can be required at times to adjust the airflow to
achieve smokeless burning and/or to reduce flare noise. Excessive airflow rates can lead to excessive flare noise.
While electrical blower motors have a limited number of starts/stops per hours, leaving a blower on after a flare
smokeless relief load has subsided can create excessive noise.
Air-assisted flare operations should consider operation of the flare system if a blower power failure occurs. Lack of
forced air allows the flare to smoke. Other considerations, such as migration of relief-gas flows into the air-delivery
system, should be considered for flare operations and design. Likewise, the radiation from a non-assisted flame
on an air-assisted smokeless flare can be significantly different from the radiation from the forced-draft airassisted flame. The highest radiation load, blower-on or blower-off, should be used for safety.
Air-assisted flare design should mitigate any possible leakage from the pressurized air-delivery system into the
flare relief-gas riser. For instance, at low relief-gas flow rates, the air pressure can exceed the gas pressure in the
flare header. If an opening, such as a tear in the gas riser or a loose gas riser flange, is encountered, forced air
can flow into the gas riser. This air can travel both directions in the flare system forming a large, potentially
combustible fuel-air mixture and an explosion hazard. This can create an explosion hazard in both the gas riser
and the upstream flare header and equipment.
A.10.5 Maintenance
Much of the blower-staging and control equipment is located where local conditions allow maintenance, provided
that access and isolation are permitted. The recommendations of the manufacturers of all equipment in the
system should provide good guidance. Such advice can includs the following.
a) The forced-draft blower and its driver should be maintained in accordance with the manufacturer’s
recommendations. This can require lubrication service.
b) Any dampers should be regularly inspected for operation and adjustment. Damper linkages can vibrate and
wear and become loose. This can upset airflow and smokeless-flare operations.
c) Controls for airflow operations should be calibrated and maintained in accordance with the instrument
manufacturer’s recommendations.
d) Any expansion joints used to connect blowers and blower ductwork to the flare stacks should be inspected
regularly for wear and leakage.
e) The air-assisted flare-burner should be observed at night-time for hot spots that can occur due to internal
burning.
f)
Flare pilots and ignition systems should be maintained as detailed in A.3 and A.4.
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publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
g) Ensure that electrical parts are not subjected to high heat or vibration.
h) Ensure that the electrical and physical integrity of control boxes is not compromised.
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A.10.6 Troubleshooting of Blower Staging and Control Systems
See Table A.7.
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Table A.7 — Troubleshooting of Blower Staging and Control Systems
Problem
Smokeless burning is not being
achieved.
Possible Cause
Insufficient airflow.
Corrective Action
Confirm that blowers and dampers are
operating correctly.
Confirm that blower adjustments are
set to use the available power.
Confirm that there is no significant air
leakage from the flare or air delivery
system.
Confirm that relief-gas flow rates and
compositions are within design
specifications.
Confirm that there is no liquid
carryover in the flare relief gas.
Excessive flare noise levels.
Excessive airflow.
Confirm that the blower, dampers and
controls are operating properly.
Flare-burner damage.
Confirm that the flare flame is stable. If
the flame is not stable, then evaluate
airflow, gas flow and loss of flameholding devices as potential causes.
Incorrect waste gas composition or
flow.
Confirm that the relief-gas flow rates
and compositions are within design
specifications and that transient flow
conditions between differing relief-gas
scenarios are not occurring.
Blower surging or flame instability.
Reduce airflow rates to see if the
excessive noise subsides. If it does, it
can be possible to advance the airflow
back to a higher flow rate to achieve
smokeless burning. Once an unstable
flame is started, it is very difficult to
mitigate without reduction of either the
gas flow or the airflow.
A.11Pressure-staging Equipment
A.11.1 Purpose
In some flare arrangements, the flare flow is designed to go to a number of burning locations (stages) to achieve
the specific object of the arrangement (often smokeless operation). See A.2 for further information. Maintaining
sufficient burner pressure during turndown conditions can be critical and often requires employing a staging
system to proportionately control the number of flare-burners in service relative to the amount of gas flowing. A.11
addresses the auxiliary equipment necessary to operate this kind of flare.
83
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publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
A.11.2 General Description
The equipment falls into three categories:
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a) Detection: The condition of the flare system is continuously monitored and the system is operated in response
to the parameters of this monitoring. This detection can be as simple as monitoring the flare-gas pressure (by
pressure switch or transmitter), but can become more complex with the inclusion of flare and steam flow
measurements with or without added modification from smoke or infrared detection (see A.11 for more
details).
b) Logic: It is necessary that the signal(s) from the detection phase be processed to result in an appropriate
control action. This can be done by local dedicated control systems (e.g. PLCs) or the logic can be passed to
a central control system (e.g. DCS). It is also common to have a local control indication of pilot and stage
condition.
c) Flow control: The control system from b) is arranged to provide the appropriate signals to operate devices
(usually control valves with on-off operation).
Figure A.17 shows a simplified control with a staged system containing three stages such as the system
discussed in A.1.5.3. The design and safety requirements of each application can impact the instrumentation
selection and arrangement.
NOTE
On some systems, the operative purpose of the above equipment can be achieved with a suitably designed liquid
seal. Such a device and its operation are described in A.7.
84
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Key
1
2
3
4
5
6
a
b
first stage
second stage
third stage
staging control
pressure transmitter
fail safe pressure relief device
Relief gas.
Fail open.
Figure A.17 — Simplified Control Diagram for a Three-stage Flare System
A.11.3 Mechanical Details
The actual selection and co-ordination of this type of equipment is very much an integral part of the proprietary
knowledge of the system supplier. The following are only general comments that can help in acquiring such a
system.
a) Reliability of the detection devices is of paramount importance; ease of maintenance is also a major
consideration. Some users address these concerns by utilizing redundant control elements such as pressure
transmitters in staging systems. Some operators prefer triple redundant instrumentation for these critical
control elements.
b) Particular attention should be paid to control logic, with the greatest importance being given to the safe
operation of the system to perform as a complete flare system.
c) The greatest concern with control valves used in this duty is that the safety of the flare system is not
compromised by a valve failure. To this end, it is common to install bypasses on such valves and these
bypasses are fitted with devices that are guaranteed to operate if the upstream pressure reaches an unsafe
level; such devices are rupture disks or any other failsafe device that is approved by a recognized pressure
relief authority. Care should also be taken so that the access and isolation of the valve systems permit
maintenance.
85
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the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
FO
d) An operational note concerning rupture disks: when a valve closes to turn off a stage, there is the possibility of
a flashback occurring in the pipework immediately after the valve. It is possible that a pressure wave
associated with this flashback can burst rupture disks from the downstream side, thus creating serious
operational problems. It is strongly urged that an after-purge with an inert material (e.g. nitrogen or carbon
dioxide) be immediately introduced into the piping downstream of the valve to clear the pipe of any material
that can form a burnable mixture in the pipe. It is not necessary that this purge be continuous, but it is
necessary that its application be at the same time, or closely following, the closing of the staging valve.
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e) Opening of rupture disks can cause a pressure wave on downstream lines and on the flare. It is necessary
that the designer consider this effect on the equipment mechanical rating.
A.11.4 Operations
As with many flare-system components, a staging system should be designed to operate automatically without
any human intervention. After commissioning, no “operations” procedure should be required (other than
maintenance and troubleshooting, see A.11.5 and A.11.6). However, any such procedure that can be required by
the system supplier, or which has been developed by site engineers after due consideration, should be suitably
promulgated and observed. A clear understanding of the automated sequence is necessary to enable an operator
to recognize (diagnose) operational misbehavior.
A.11.5 Maintenance
Much of the staging-system equipment is in a position where local conditions allow maintenance, provided that
access and isolation are permitted. The recommendations of the manufacturers of all equipment in the system
should provide good guidance. Such advice can include the following.
a) API Standard 521 describes the maintenance and troubleshooting for flare knockout drums. Adequate
knockout-drum capacity should be installed immediately prior to the flare for any application that deals with a
gas composition that can approach its dew point. Liquid results in smoking flames and a significant growth in
flame length that can produce severe damage to the surrounding equipment.
b) Stroke all staging valves fully open and then rapidly close on a routine basis. A common practice is every two
to three months. This testing ensures the valves function and prevents seizure of the seating surfaces and
actuators.
c) Check the bypass device or the staging valve for leakage. One method commonly used for this confirmation is
to install a block valve ahead of the device and a small connection for the introduction of nitrogen. The
operator confirms that pressure produced via the introduced nitrogen is maintained for a set period of time,
indicating that the system is gas-tight.
d) If pin-actuated devices are installed, flex the pin moving the piston on a routine basis, sufficient to prevent
seizure of the O-rings to the valve body.
e) The instruments associated with the staging and pilot systems should be treated as critical safety controls and
inspected and calibrated on the schedule the plant has established for such critical control elements.
f)
Refer to A.3.5, A.4.5 and A.5.7 for the maintenance of pilots, ignition systems and pilot-detection systems.
g) Ensure that the scaffolding, debris, etc., do not impair the operation of any valve or similar part.
h) Ensure that the electrical parts are not subjected to high heat or vibration.
i)
Lubricate actuators, as appropriate.
j)
Ensure that the electrical and physical integrity of control boxes is not compromised.
k) Replace burned-out indicator bulbs promptly.
A.11.6 Troubleshooting
See Table A.8.
86
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the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
Table A.8 — Troubleshooting of Pressure-staging Equipment
Problem
Relief device has blown and the flow
reduced to a low level: Smoke occurs
on a stage that is now no longer under
the control of the staging system, i.e.
the relief device is an open pipe to the
affected stage.
Corrective Action
Restore relief device to closed
condition.
Arrange control scheme to indicate
opening of relief device (possibly using
limit switches to indicate correct
operation).
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Smoking or flames on a stage that
should be closed.
Possible Cause
Smoking or flames on a stage that
should be closed.
Leaking valves or bypass devices.
Repair valve or bypass device.
Smoking or flames on a stage that has Normal behavior for a short time while
just closed.
the residual gas downstream of a
staging valve bleeds off.
If the condition does not correct itself
in a few minutes, check for a blown
relief device (possibly using limit
switches to indicate correct operation).
Use a post-purge system to flush
residual gas out of the affected stage
more rapidly.
Smoking on a stage that should be
open.
Incorrect waste-gas composition.
Route sources of this composition to
another flare system or stand-by
emergency flare.
Replace burners with equipment
designed to handle this gas
composition.
Smoking; stage valve does not close
at the time expected.
Insufficient air pressure to valve
actuator.
Check for obstructions in the
instrument air line, such as a closed
block valve, kinks in tubing, failed
pressure regulator, etc. (possibly using
limit switches to indicate correct
operation).
Pulsing flames.
Staging system can enter a mode of
frequent opening and closing
(“fluttering”) at certain flare loads.
Modification of the software controlling
the valve action by adjusting response
rates, modifying timing delays, etc.
A.12Flow and Pressure Sensing Equipment
A.12.1 Purpose
A flare system may be fitted with a flow and/or pressure measuring system for a number of reasons, including:
a) to act as an input element into a smoke control system or
b) to provide record-keeping for the total flow going to a flare for operational or legal reasons.
A.12.2 General Description
The selection of equipment for this duty is dependent on its primary purpose. Although multiple parameters may
be measured by such instrumentation, there are generally two types of systems.
a) Pressure: It is common to measure the pressure in the type of flare that uses the pressure of the gas to
provide the energy necessary to give a smokeless performance. This pressure input provides the indication of
flare-system condition to operate the pressure-staging system described in A.11. The measuring element can
be a pressure switch, but pressure transducers are being used increasingly.
b) Flow: A flow meter is an alternate and more direct way to determine flow. However, the measurement of
volume alone does not take into account any variation in its relative molecular mass. Certain installations can
require a mass flow meter to achieve the desired objectives. A system that can have flows of compositions
87
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the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
with varying degrees of saturation can require more than just a measure of mass flow as an input for smoke
control.
A.12.3 Mechanical Description
FO
The mechanical details of equipment covered by A.12 are very much dictated by the requirements of the
instrument supplier. There are, however, some general points that should be observed.
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a) In as much as the flare system, in general, should be available at all times, the application of plant-wide
installation specifications might not be applicable. As an example, whereas an instrumentation specification
can demand that isolating valves be fitted on all pressure-sensing devices (for service purposes), it should be
recognized that a flare system can depend upon the assumption that a pressure-sensing switch or transmitter
is always live to the process and it cannot be turned off except under controlled circumstances.
b) The potential flow range in a flare system is extremely wide. It is necessary to take care to ensure that
equipment designed for a normal, relatively small flow rate is not damaged or upset by occasional high-flow
(and -pressure) excursions. Conversely, equipment capable of measuring very high flows can have difficulty
measuring low flows with sufficient accuracy for efficient day-to-day operation.
c) Flare streams are notoriously unpredictable in composition. This affects the interpretation of flow information
(e.g. for smoke control, a flow meter might not be able to differentiate propane from propylene).
Unpredictability can also affect the physical operational condition of a system (e.g. it is common to have
gummy liquids and solids existing in flare lines that can render flow-detection elements inaccurate or
inoperative).
d) Equipment reliability should be carefully assessed in reference to the importance of the function of the
equipment.
A.12.4 Operations
Under normal circumstances, this equipment should operate automatically.
A.12.5 Maintenance
The manufacturer’s recommendations should be used as a guide for maintenance. The opportunities for
maintenance can be restricted, however, and this should be assessed at original installation [see A.12.3 a)].
A.12.6 Troubleshooting
The manufacturer’s recommendations should be used as a guide for troubleshooting.
88
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publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
Annex B
(informative)
Components of Multi-burner Staged Flare Equipment
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B.1 Burners
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A multi-point flare has multiple burners to distribute flared gases across several burning points. The multiple
burning points may be arranged in arrays located near grade or at an elevated position. See Figure B.1 for one
example of each.
Multi-burner staged flares are fed from a manifold. The manifold distributes the flow of flare gas to individual
branches containing one or more burners. On/off valves direct the flow of flare gas to each branch. The on/off
valve feeding each stage opens or closes depending upon the upstream pressure. See Figure A.17 for a
simplified control diagram of a three stage control system.
A burner ejects the flare gas to entrain surrounding air at sufficient velocities to induce proper mixing for ignition
and stable combustion. An individual burner has multiple orifices and some means of flame stabilization. The
burner can be constructed of cast or wrought materials.
Typical burner metallurgy is high-grade, austenitic stainless steel. The burners are normally welded to the flaregas risers. In some cases, they are threaded, then back-welded.
Burner riser material should be of suitable grade to withstand operating flare and flare-gas temperatures. Flaresystem operation may include situations when releases become cryogenic. Typical material for the riser is
type 304 stainless steel in the upper portion [approximately 1.8 m (6 ft)]. The lower portion of the riser can be of
carbon steel. It should be properly insulated and jacketed, if required.
a) Grade type
b) Elevated type
Figure B.1 — Multi-burner Staged Flares
Burner performance is dependent upon burner spacing and elevation and on row spacing and length. These
factors influence air supply to the burners. Proper burner spacing is critical for reliable cross-lighting between
burners of a given stage.
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the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
B.2 Pilots
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B.2.1 Flare pilots are expected to reliably ignite the individual flare stages. Cross-lighting ignition from adjacent
stages is not recommended. If an individual stage pilot fails to operate in correct stage sequence, unburned
hydrocarbons and/or toxic gases can be released to the atmosphere, potentially resulting in a vapor-cloud
explosion, odor problems or adverse health effects. Should a flare pilot fail to ignite a particular stage in proper
sequence, subsequent stages can be prevented from operating. In most multi-burner flare applications, the pilot
cannot be accessed for service or replaced while the flare is in operation. Consequently, the pilot system should
be reliable enough to operate for years without maintenance.
B.2.2 Pilot designs are similar to those for elevated pilots. The majority can be described as fixed heat-release,
self-inspirating, pre-mix burners. See A.3 for additional details.
B.3 Ignition Equipment
B.3.1
The purpose of the ignition equipment is to reliably ignite the pilot.
B.3.2
There are four types of ignition systems that are commonly employed to light flare pilots:
 spark ignition at pilot tip,
 spark ignition of a portion of the pilot gas/air mixture prior to the pilot tip,
 compressed-air flame-front generator,
 self-inspirating flame-front generator.
B.3.3 No single ignition system is preferred in all circumstances. For improved reliability, multiple ignition
systems are often installed. See A.4 for additional details.
B.4 Flame-detection Equipment
B.4.1
The purpose of the flame-detection system is to confirm that the pilots have ignited and are burning.
B.4.2 There are several types of flame detection such as thermocouples, flame-ionization detectors, optical
systems and acoustic systems. See A.5 for additional details.
B.5 Buoyancy and Velocity Seals
Buoyancy and velocity seals are typically not applicable to multi-burner staged flares.
B.6 Manifolds
A common manifold distributes the flow of flare gas to individual manifolds that contain multiple burners. The
manifold materials should be of suitable grade to withstand operating flare and flare-gas temperatures. The
individual manifolds may be externally insulated or covered by earth and stone, as appropriate.
Open/close valves supply flare gas to individual manifolds. Figure A.9 shows the effect of opening additional
stages as the flow increases. Should the open/close valve fail to operate as required to prevent overpressurization, a valve-bypass piping system may be utilized.
After the closure of each stage, the piping downstream of the staging valve may be purged. An inert gas may be
used to purge the line of any combustible gases.
B.7 Operations
A staging system is required to operate automatically without human intervention. After commissioning, a no
“operations” procedure is required (other than that described in B.8 and B.9), but the system supplier’s advice
should be followed.
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publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
B.8 Maintenance
FO
Much of the staging-system equipment is in a position where local conditions allow maintenance, provided that
access and isolation are permitted. There is nothing about the use of any such equipment that is specific to flare
systems and, thus, the recommendations of the manufacturers of all equipment in the system should provide
good guidance.
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Maintenance items for a multi-burner staged flare system are primarily those associated with the staging system.
A clear understanding of the automated sequence is necessary to enable an operator to recognize (diagnose)
operational misbehavior. Refer to A.11.5 for a list of maintenance items.
B.9 Troubleshooting
Troubleshooting of multi-burner staged flare systems most often involves troubleshooting the staging system.
Refer to A.11.6 for this troubleshooting guide.
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Annex C
(informative)
Enclosed-flame Flares
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C.1 Purpose
There are circumstances when it is desirable that all or part of a flare load be disposed of in a way that causes the
minimum of disturbance to the immediate locality including:
a) to eliminate or reduce radiant heat to nearby equipment or work areas,
b) to reduce noise in the immediate vicinity,
c) to make the flare flame less obvious for community relations,
d) to potentially achieve improved emissions.
C.2 General Description
C.2.1 Overview
Enclosed-flame flares burn the flare gas from a burner or burners placed as near to the ground as is practicable to
ensure good operation. The resulting flames are hidden from sight by a surrounding wall or chamber. The top of
the chamber is open to the atmosphere and allowance is made in the bottom of the chamber to permit the ingress
of combustion air. It is common for the chamber to be surrounded by a wind fence to reduce the effect of
crosswinds on the combustion process and to prevent unauthorized access.
An enclosed-flame flare system has a number of key components, including:
 a combustion chamber,
 burners,
 piping systems,
 a wind fence,
 operational and safety controls.
An enclosed-flame flare is more complex than simply installing a pipe flare inside a combustion chamber. This
flare design requires an engineered combustion process, with considerations for airflow into the combustion
chamber and flue-gas flow from the chamber. Burner designs have been specially developed to meet the
combustion requirements of enclosed-flame flares.
Enclosed-flame flares are typically rated for normally occurring flare-relief conditions. For selected applications, an
enclosed-flame flare is the first stage of a flare system that includes another flare for the combustion of larger,
emergency flare-relief flows. See Figure C.1.
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publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
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Key
1
2
3
4
5
a
elevated flare
liquid seal
enclosed flare
staging-control system
burner
Relief gas from plant.
Figure C.1 — Typical Enclosed Flare Staged to Elevated Flare
C.2.2 Combustion-chamber Size and Shape
Since the combustion chamber encloses the flare flame, it is necessary that consideration be given to the size of
the flame. The flame size is a function of the burner design, the air-side pressure drop and the burner gas
pressure. Increasing draft and relief fuel pressure helps reduce flame size.
3
Combustion-chamber design typically results in a volumetric heat release of about 310 kW/m (30,000 Btu/h/ft3).
The design volumetric heat release is a function of the burner size and design, combustion-chamber geometry,
relief-gas composition and other factors. The largest enclosed-flame flares now operating are rated at capacities
above 120,000 kg/h (270,000 lb/h). The smallest enclosed-flame flares are rated at only a few hundred pounds
per hour of relief-gas flow.
NOTE
Other considerations include open surface heat release for an enclosed flare, getting air into the center of the
enclosed flare on larger sizes and the amount of air that should be entrained as part of the combustion process, i.e. excess air.
The combustion chamber can be configured in several shapes, including vertical cylindrical, rectangular and multisided. The choice of shape includes a number of process, safety, structural and economic considerations. Site
fabrication and economic factors are often paramount in the selection of the shape of an enclosed-flame flare.
The top of the combustion chamber is open for flue-gas discharge. None of the combustion chamber shapes has
an inherent advantage in the mitigation of combustion noise. See Figure C.2.
93
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publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
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Key
1
combustion chamber
2
3
refractory lining
flare-gas burners
4
5
pilots
crushed rocks/gravel
6
wind fence
Figure C.2 — Enclosed-flame Flare
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publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
C.2.3 Mechanical Considerations
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The shape and size of the combustion chamber impacts the degree to which the flare can be pre-assembled to
meet field-erection requirements. The external shell of the combustion chamber is typically fabricated of carbon
steel. Internal surfaces can be coated or painted to mitigate dew-point corrosion as required by the enclosedflame flare design and operating conditions. Any coating material should be compatible with the design metal
temperatures for the flare and its refractory lining. Special consideration should be given to selection and
application of protective coating systems since metal temperatures can exceed temperature limitations of
standard protective coating systems [typically 205 °C (400 °F)].
Ladders and service platforms, for access to enclosed-flame flare instruments and for stack emissions sampling,
can impact the structural design of the combustion chamber.
The internal floor is usually made of compacted earth or gravel. No paved area is used, unless protected from
flame radiation.
C.2.4 Burners
Burners and burner-control systems for enclosed-flame flares are engineered for specified gas flow rates,
pressures and compositions. Burners can be, natural draft (unassisted), steam-assisted, air-assisted, or use the
relief gas pressure (pressure-assisted) to produce smokeless burning and to assist in control of the flame profile.
Burners may have to be fuel-gas enriched to achieve desirable combustion efficiency for low-heating-value and
hard-to-combust relief gases.
Burners typically combust a variety of gas compositions and at a variety of flow rates. Staged and unstaged
burner systems necessitate different design considerations. With a staged burner system, typically the first stage
turns down to purge flow rates. This burner staging provides control of the gas discharge pressure to ensure
proper mixing of fuel and air and thus control of the flame profile. Typically a large gas flow rate at a low-pressure
discharge produces a soft, large flame, unless supplemented by energy from the combustion airflow or assist
media. Such flames can be difficult to contain in the combustion chamber and have a propensity to produce
smoke or visible flame from the combustion chamber
All enclosed-flame flares, except the very smallest in size, use multiple burners. For larger-capacity enclosedflame flares, the multiple burners typically operate in staged systems as is described in Section 6.
The design of burners for enclosed-flame flares is proprietary to the manufacturer.
Airflow design into the combustion chamber establishes the distribution and velocity with which the air mixes with
the fuel discharge. A pilot flame initially ignites these fuel-air mixtures. Once a main-burner flame is established,
the burner should be stable and maintain continuous flame ignition on its own. It should never be necessary for
the flame stability of a main burner to rely on the pilot flame. Burner-flame stability is produced by the flare
manufacturer’s proprietary means. Mechanisms include mechanical elements of the burner design in conjunction
with air and gas flow dynamics.
C.2.5 Burner Piping
It can be necessary to protect piping that is internal or external to the combustion chamber, but within the wind
fence, from radiant heat loads by radiation protection. When there is a possibility of condensation and liquid
drainage into the combustion chamber resulting in flame impingement on internal piping, the piping should be
engineered for high-temperature exposure. Piping is typically covered with loose gravel or metallic shielding. The
covered piping should be suitably protected from environmental effects and corrosion.
If liquid carryover and/or gas condensation can occur, the piping design should accommodate liquid removal (e.g.
sloping piping to knockout drum).
C.2.6 Pilots
Pilot fuel and supply systems should be clean and reliable. The typical pilot is a fixed-firing-rate, pre-mix burner.
The pilot gas orifice is generally quite small. Potential plugging of this orifice should be mitigated by good piping
design and by the use of a strainer located immediately upstream of the pilot gas orifice.
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publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
Pilots for enclosed-flame flares can be engineered to facilitate inspection and maintenance while the flare remains
in service. This is accomplished by locating key components external to the wind fence and/or by making the pilot
assemblies easily removable from outside the wind fence.
Many of the pilot and pilot-ignition details covered in A.3 and A.4 are applicable to enclosed-flame flares.
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Flame scanners may be used as the flames are enclosed in the combustion chamber. Pilots for burners in
enclosed-flame flares are typically more protected from the weather than those of open-air elevated flares. With a
properly designed enclosed-flame flare and with an effective wind fence design, the airflow across the pilot and
burner is unidirectional whereas open-air elevated-flare pilots are affected by wind from varying directions.
C.2.7 Air and Flue-gas Flows
An enclosed-flame flare design provides for the airflow into the combustion chamber and for the flow of hot flue
gases out of the combustion chamber. The heat produced in the combustion process is absorbed by large
quantities of excess air so that the resulting flue-gas temperature is low enough to allow the use of common
refractory materials. The airflow into the combustion chamber can be by natural draft or forced draft. Natural draft
is most often employed on large-size enclosed-flame flares. The natural draft level produced at any flaring rate is
a function of:
 the heat release and resulting flue-draft gas temperatures,
 the airflow dynamics into the combustion chamber,
 frictional and combustion pressure losses as the flame propagates,
 the flow throughout the combustion chamber,
 the pressure loss of the flue gases exiting the combustion chamber,
 the combustion chamber dimensions and
 air fuel ratio controls (if equipped).
These factors can be engineered for the performance of the enclosed-flame flare from minimum flow rates to
maximum flow rates. The maximum operating temperature of the combustion chamber is set by such engineering.
For natural-draft enclosed-flame flares (without air control) at gas flow rates less than maximum, the combustionchamber temperature is lower and its operating excess air is higher. Even with cooler flue-gas exit temperatures
and higher overall excess-air levels, high combustion efficiencies are achieved from the flames alone, as is the
case for most open-air elevated flares.
The enclosed-flame flare may achieve higher combustion and destruction efficiencies with the flare flame
contained in the combustion chamber. Dampers or other means may be used to control the natural-draft airflow
into the combustion chamber. Control of the airflow can allow for control of the combustion-chamber operating
temperature over variations of relief-gas flow rate and composition. Controlled combustion temperature may
achieve higher hydrocarbon destruction efficiencies.
Forced-draft air movement may be used for multiple purposes in enclosed-flame flares.
a) Some air-assisted designs use forced air to supplement the flame energies to produce smokeless flames of
reduced flame volume.
b) Designs for 100 % forced-draft air volumes are controllable and thereby, control the enclosed-flame flare
combustion-chamber temperature.
Excessive use of forced draft can contribute to enclosed-flame flare noise, resonance and vibration. The use of a
forced-draft fan and its driver impacts the reliability and availability of the overall system and should be evaluated.
Flue-gas flows from the enclosed flare occur at the temperature of the combustion chamber for the given
operating conditions. Typically, the temperature factor dominates for the dispersion of combustion products. If the
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publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
flame volume is contained within the combustion chamber, there is very little, if any, measurable thermal radiation
from the plume. However, the hot plume from the flare can impinge upon structures and components that are
close by and above the elevation of the combustion-chamber discharge. Design considerations for local structures
include: distance, height, dispersion and exposure for personnel. This is done on a case by case basis.
FO
C.2.8 Wind Fences
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Enclosed-flame natural-draft flares use wind fences or other designs to mitigate the potential of the wind to upset
air and flue-gas flows. Uniform airflow to all sides of all burners is important in achieving controlled combustion.
Wind fences surround the burner air inlets and are designed to allow distribution of the airflow to the burners.
Without a wind fence, the wind can upset the natural draft of the combustion chamber. This can result in flames
exiting the base of an enclosed flare. Without a wind fence, the wind can flow in the upwind burner openings and
out of the downwind ones. It is necessary that wind-fence design consider the enclosed-flame flare operating draft
levels; care should be taken not restrict airflow outside the intended design to the burner openings. See
Figure C.3.
Wind-fence designs acoustically dampen the noise. See C.3.2.
Wind fences also offer safety protection for personnel from the radiation of the flare flames and from the external
surfaces of the combustion chamber. The inside surface of the wind fence and all components of the enclosedflame flare inside the wind fence should be engineered for the temperatures that are experienced from the thermal
radiation of the flames visible there. Personnel access inside the wind fence of an operating enclosed-flame flare
should be restricted.
The wind fence also isolates the air intake for the enclosed-flame flare from the adjacent ground-level
environment. Elevating the air intake can mitigate the possible ignition of combustible ground-level hydrocarbon
vapor clouds. This is an important factor where the enclosed-flame flare is located in close proximity to
hydrocarbon storage or processing equipment.
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Key
1
exterior frame
2
3
wind fence
refractory lining
4
5
flare-gas burners
gas pilots
Figure C.3 — Wind Fence for an Enclosed-flame Flare
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the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
C.2.9 Operational and Safety Controls
Enclosed-flame flares require a number of operational and safety controls. As for any flare, the relief gas should
never be ignited without the assurance that safe operating conditions exist. A prerequisite is that the flare system
is purged and all flare safety systems are operational.
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Some flare-burners in a stage utilize pilots and pilot ignition systems. A flame-front generator or direct electric
ignition of the pilots is often employed. Flame-detection devices monitor pilot flames. It is necessary that
automatic pilot re-ignition be incorporated into the system design. Operating pilots are necessary to allow flareburner staging.
A burner staging system, as is described for the burners above and in Annex B, is used on larger-capacity
enclosed-flame flares.
The combustion chamber of an enclosed-flame flare can overheat if the gas heat release is too high and/or if the
airflow is not sufficient. The gas heat release can be too high due to excessive gas flow or due to changes in gas
composition. The airflow demand can exceed the design or can become restricted. A high-temperature alarm
and/or shutdown should be supplied to protect the combustion chamber. It is important to ensure that the possible
shutdown of the fuel train of an enclosed-flame flare does not restrict safe discharge and disposal of relief gases.
The high-temperature control action can disengage a burner stage and effect a diversion of the relief gases to
other systems such as an elevated flare.
The enclosed-flame flare system can require a purge or sweep gas. As is typical for staged burner systems, only
the first stage can require a purge-gas flow. Some smaller enclosed-flame flares eliminate purge gas flows by
opening and closing the first-stage burners to maintain a minimum pressure in the flare header. For relief-gas
compositions with a wide ratio of upper to lower flammability limit, an inert-gas post-purge of a burner stage as it
turns off is recommended. The post-purge sweeps the reactive gas out of the burners and burner piping, and
mitigates flashback and combustion in the flare-system piping. Gases of concern include hydrogen, ethylene,
acetylene and others as defined by a high ratio of upper to lower flammability limits.
Enclosed-flame flares can be tested for combustion performance. Flue gas can be sampled in the combustion
chamber or an extractive sample can be drawn out of the combustion chamber. The emission factors for an
enclosed-flame flare can, thus, be measured. The ability to measure can be significantly influenced by the
physical configuration of the flare.
Relief-gas compositions that are difficult to ignite and combust can be aided by the use of fuel enrichment. Flaregas analyzers combined with control systems can be implemented for fuel-gas enrichment. With temperature
control in an enclosed-flame flare, less enrichment gas is required to achieve higher combustion/destruction
efficiencies than are typical for an elevated flare.
For enclosed-flare flames that are located in an area where gas vapors can be present, lower explosive limit (LEL)
meters with an alarm should be located adjacent to the flare. Alternate choices are to shut the flare down and/or
to divert the flare gases.
C.2.10 Enclosed-flame Flare Applications
Enclosed-flame flare applications include:
 flares for normally occurring relief rates for hydrocarbon processing and production facilities such as startup/shutdown flows and normal process venting;
 petroleum terminal vapor control;
 biogas disposal: the products of anaerobic digestion (e.g. from landfills, industrial digestion processes or
sewage processing) are fed at a fairly steady and predictable rate;
 flare applications where combustion-chamber temperature control ensures a high hydrocarbon-destruction
efficiency;
 flare applications where the assist fuel-gas quantity can be reduced by use of an enclosed flame;
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 in refining or petrochemical applications where the flare acts as a lower stage to the complete relief system,
designed to handle day-to-day loads (see Figure C.3);
 onboard floating-production storage and off-loading (FPSO) vessels, where the bulk of the flaring events are
handled in a safe way in the confined space available.
FO
C.3 Operating Considerations for Enclosed-flame Flares
C.3.1 Visible Flames
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The purpose of the enclosed-flame flare is to hide the flame. Visible flame can be caused by the following:
 exceeding design heat-release capacity;
 undersized combustion chamber;
 burner performance or operation related to:
 control of smoke-suppression medium,
 burner arrangement/position,
 burner plugging or damage, and
 liquid carryover to the burner;
 air distribution to the burners and combustion chamber;
 wind effects;
 poor temperature control for units operating with a temperature-controlled combustion chamber.
In some cases, reported flame visibility is simply reflected light from the combustion chamber on a foggy or lowcloud night. High combustion-chamber temperatures can produce a visible, ionized gas glow of the flue products
exiting the combustion chamber that can appear to be visible flames.
During normal flare operation up to the maximum capacity of the units, it is necessary that the flame length be
contained within the enclosure and not be directly visible from the outside. For the majority of specified operating
cases, combustion is smokeless.
The flare should be designed to mechanically withstand certain overload cases for short duration. These cases
cause a greater or lesser amount of flame to come out of the top of the enclosure and be visible to a remote
observer. Generally, operating in an overload condition is discouraged.
C.3.2 Noise and Vibration
As some heat release energy in an enclosed-flame flare is converted to acoustical energy, high noise levels can
be encountered. Burner design and burner stability are key elements to controlling enclosed-flame flare noise,
with the following considerations.
a) Burners of moderate gas/air mixing intensity avoid creating excessive noise with typical volumetric heat
release.
b) If burners of greater flame intensity are utilized, the ground flare has an increased tendency to produce
excessive combustion-driven noise.
c) If burners of less intensity are used, the enclosed flare can be quieter since the combustion chamber is
proportionally physically larger in size.
The combustion chamber can amplify any noise produced by unstable burners or unstable gas or airflow.
Excessive low-frequency noise and vibrations can be encountered if a resonance is set up in the combustion
chamber. Typically, the combustion-chamber prime resonant frequencies are sub-audible. This low-frequency
noise can travel significant distances without attenuation and can induce vibrations in structures remote from the
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the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
enclosed-flame flare. Resonance problems are best avoided by empirical experience. If a problem does occur, the
most readily available remedy is modification of the burners and burner operating systems and/or a reduction in
operating capacity.
FO
Noise levels from an operating enclosed-flame flare are a function of heat release and equipment design. Noise
levels are affected by the design factors listed above, including the number of stages that are operating. Windfence designs can serve to acoustically isolate the combustion-chamber noise. Some flares can achieve an
85 dBA noise level or less at a distance of 0.9 m (3 ft) from the wind fence.
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C.3.3 Refractory Failure
Refractory failures can result in hot spots on the shell of the combustion chamber. How the refractory fails, the
nature and extent of its failure and its consequences and repair are a function of the type of lining used.
For ceramic-fiber linings, shell hot spots often develop initially at the seam of the blanket lining where hightemperature contraction has opened a gap. This is avoided by proper design of the refractory lining that considers
such shrinkage. Ceramic-fiber shrinkage rates can increase when subject to cyclic service and proximity to flames
in enclosed-flame flares. Ceramic-fiber lining can also fail due to over-temperature and/or excessive-velocity
operations. For high-velocity failures, particles of the high-temperature lining can be discharged from the top of
the combustion chamber. High-temperature, high-velocity failures are avoided by proper material selection, proper
anchoring design, good installation and by good operating and maintenance practices. When using rigidizers to
improve the velocity rating of ceramic-fiber linings, consider the cyclic temperature operation of enclosed-flame
flares and thermal expansion difference of the rigidized material and the base material. Ceramic-fiber linings
should be avoided on horizontal surfaces where liquid hydrocarbons can collect. If a pool fire develops on a flat
horizontal surface, the fiber material’s insulating capabilities can be significantly reduced.
For castable or other hard material linings, hot spots on the combustor shell typically occur first at expansion-joint
or seam locations. These are avoided or mitigated by eliminating expansion joints where practical and/or by
proper expansion-joint design and maintenance. Castable-type refractory materials are also subject to failure by
reason of improper initial curing. Castable refractory should be cured in accordance with the manufacturer’s
recommendations. Cosmetic cracks produced during curing/initial operation can be expected and generally do not
1
affect long-term performance. Larger cracks that are 3 mm ( /8 in.) or greater in width and penetrate more than
50 % of the castable thickness should be considered unacceptable and be repaired. See API Standard 560
regarding repair techniques. Some phosphate-based castable refractories do not require a high-temperature
bake-out. Castable-refractory strength and durability can be enhanced by the addition of metal needles.
Polypropylene fibers have been successfully used to enhance the thermal cycling and cure-out for castablerefractories.
It is necessary to include a proper anchoring means in any hard refractory system. Repairs to hard refractory
systems should be made in accordance with the manufacturer’s recommendations.
The use of high-temperature alarms and shutdowns can mitigate some refractory failures.
C.3.4 Pre-commissioning
Pipework associated with the flare should be tested prior to the installation of the flare-burners and pilots, with
consideration of the following.
a) All flare lines should be free from debris and obstruction. All lines should be blown down prior to installing the
flare-burners, pilots and steam nozzles (if fitted). All lines should be blown down with a velocity greater than
that which is encountered during normal operation. Typically, such velocity exceeds 90 m/s (300 ft/s).
b) Ensure that the pilot orifices are not blocked.
C.3.5 Commissioning
C.3.5.1
Initial Commissioning
When initially commissioning the flare header or following any shutdown where the flare header is gas-free and
positively isolated, the following procedure is applicable.
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a) It is necessary that all scaffolding, supports, tools, etc. be removed from within the perimeter of the wind fence
or other barrier that indicates restricted access.
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b) The flare line downstream of the main header blind should be purged with inert gas to reduce the oxygen
levels to safe proportions. The header should be purged with at least 10 times the free volume of the header
with a non-condensable, inert gas. As a result of this purge, a maximum oxygen concentration of less than
6 % volume fraction is recommended, unless process conditions indicate a more conservative level should be
reached. The use of inert gas as the purge medium prior to pilot ignition precludes the possibility of a gas/air
mixture forming within the flare enclosure that can ignite explosively when the pilots are ignited. After the
pilots are ignited, a hydrocarbon gas purge can be used.
c) In consideration of the inert-gas purge, normal safety precautions should be taken within the flare area.
C.3.5.2
Bringing the System Online
When bringing the system online, the following procedure is applicable.
a) Remove blinds from the steam line, if appropriate. Slowly admit steam to distribution pipework in a manner
that avoids excessive condensation and water hammer. Check functioning of steam traps and any flexible
hoses.
b) Prior to pilot ignition, ensure that the oxygen content of the flare header has been maintained at less than 6 %
volume fraction.
c) Ignite pilots in accordance with the manufacturer’s instructions.
d) Verify pilot ignition.
e) Remove blinds from the main headers.
The system can now be considered online.
C.3.6 Normal Operations
C.3.6.1 Multiple-flare Operations
For large plant applications, an enclosed-flame flare is typically designed for flow rates at normally occurring plant
operations. In some plants, multiple enclosed-flame flares are utilized upstream of the emergency flare.
Emergency flaring still requires a larger-capacity open-air flare. How the multiple flares operate, e.g. in series or in
parallel, is dependent on meeting the requirements of the engineered equipment and controls of the plant. Further,
the staging of flares can maximize the use of the capacity of the enclosed-flame flare. Finally, if an emergency
flare relief occurs, all flare-gas flow can be directed to the emergency flare.
Safe multiple-flare operation requires the provision of a means to prevent very low relief-gas flows from going
unimpeded to multiple flare locations. If low relief-gas flows have more than one possible exit point, it is quite
likely that all of the gas can flow to one flare and the air ingress through the other flare(s). Such flare cross-flows
are prevented by the use of flare-staging devices, including liquid-seal and valve-operated staging systems and
flare purge systems.
One advantage of having a primary enclosed-flame flare is that it mitigates the internal burning and external flame
impingement on a large flare sized for emergency flaring loads when lower-level flaring occurs. The service life of
the emergency flare is greatly extended. If the primary enclosed-flame flare requires maintenance, this can be
accomplished by diverting all flows to the emergency flare. The primary flare, thus, does not require a plant shut
down for maintenance. By reducing the potential damage to the emergency flare during lower-level flaring
conditions, the likelihood of requiring a crane for its maintenance is reduced.
Effective operations on an enclosed-flame flare greatly reduce the visibility of flaring occurrence, thereby
improving plant-community relations.
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C.3.6.2
Hydrocarbon Purge
The hydrocarbon purge gas flowing from the burners is normally ignited from the pilots. If, at any time, the flame is
visible or is excessively long, then the purge rate should be checked and adjusted.
FO
Combustible purges can produce internal burning, which is indicated by smoke exiting the burners. This problem
can be corrected by adjusting the purge rate. This problem can also be caused by a failure of a staging device.
C.3.6.3
Inert-gas Purge
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Using inert gas for the purge medium is highly preferable for start-up and is optional for normal operations. It has
the advantages of having no detrimental effect to the burners and of not affecting pilot operation.
C.3.6.4
Normal Flare Operation
Any flare gas that passes to the enclosed-flame flare is distributed to flare-burners, often through a valving system
that responds to demand.
During normal flare operation up to the maximum capacity of the units, the flame length should be contained
within the enclosure and not be directly visible from the outside. For the majority of specified operating cases,
combustion is smokeless.
The flare may be designed to mechanically withstand certain types of overload for short duration. These cases
cause a greater or lesser amount of flame to come out of the top of the enclosure, making the flame visible to a
remote observer.
C.3.6.5
Normal Shutdown Procedure
The normal shutdown operation is as follows.
a) Allow flare-gas flow to decline to zero.
b) Shut off pilot gas.
c) Shut off purge gas.
d) Install line blinds as appropriate before commencing maintenance.
C.4 Maintenance
Depending on the design of the flare and its ancillaries, some maintenance work can be possible during the
operation of the flare. Any valve-staging equipment, as well as instruments and devices mounted on the outside
shell of the flare enclosure, is likely to be accessible. Normal inspection and maintenance procedures specified by
the manufacturer or that are good practice should be followed.
General inspections of all aspects of the flare should be undertaken at every convenient shutdown. In particular,
the following conditions should be assessed:
a) general burner condition:
1) check for distortion/damage,
2) check for condition of feeder piping;
b) carbon deposits: remove any excessive deposits;
c) port blockage: blow clear, as appropriate;
d) pilot burners:
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1) nozzles should be cleaned,
2) orifices should be cleaned;
e) refractory lining:
FO
1) the internal lining should be examined visually and an assessment made on the level of damage at every
suitable opportunity when the flare is shut down; see C.3.3 regarding expansion-joint condition and
cracking considerations,
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2) temporary patching should be considered to avoid further damage,
3) visually inspect for hot-spot distortion during an outage,
4) inspect for hot spots while operating, either visually or by using infrared means;
f)
structure and manifolding: flare structures and burner manifolding should be examined using normal
maintenance procedures and action taken as appropriate;
g) staging and block valves: valves should be regularly stroked to ensure continued operation; actual valve
position should be compared versus the intended position from control signal; valves should be maintained in
accordance with the manufacturer’s instructions.
C.5 Troubleshooting
See Table C.1.
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Table C.1 — Troubleshooting of Enclosed-flame Flare Systems
Problem
Possible Cause
Corrective Action
Several
See A.4.6 for general guidance.
High-frequency noise.
Most likely associated with steam
injection
Check steam quantity and properties.
Combustion roar (low frequency).
Intense combustion.
a)
b)
Check flare-gas pressure.
Check steam quantity.
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Pilot failure.
Backfire.
Flashback in stage manifold.
Check after purge system is operative.
Smoke.
1.
Air starvation.
a)
b)
Check for wind-fence blockage.
Is the wind condition unusual?
2.
Low gas pressure.
a)
Check bypass relief devices
(e.g. rupture disks).
b)
Check staging valve and system
operation.
a)
Check steam supply and/or
blowers.
3.
Visible flame.
Steam/support-air shortage.
Excessive flow.
Check diverting water seal or valve.
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Annex D
(informative)
Offshore Flare Systems
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D.1 Design Considerations for Offshore Flare Systems
D.1.1 Location Considerations
A platform mounted flaring system needs to be designed to produce acceptable thermal radiation levels, noise
levels, dispersion of unignited vapor and thermal plume away from all parts of the platform during all possible wind
conditions and for the full range of flaring scenarios.
NOTE
It is important to distinguish between wind speed for thermal radiation and structural design. Refer to
API Standard 521, Sixth Edition, 5.7.2.
The key differences between offshore and onshore flare design are the space restrictions, the proximity of living
quarters and working areas relative to the flare.
Design considerations for the flare location include the following.
a) Thermal radiation during emergency and continuous flaring events.
b) Radiation shields are used more often offshore than onshore due to the space constraints unique to offshore.
However, the onshore design acceptance criteria for thermal radiation apply offshore (see API Standard 521,
5.7.2.3).
c) Dispersion of combustion products (e.g. SO2 if sour), unignited dispersion (cold vent scenario with flame out).
d) The need for smokeless flaring during regular maintenance flaring, flare ignition and re-ignition options for the
environment (including any host country limitations), noise, helicopter approach, maintenance access, and
platform structural design.
e) Wind direction and speed:
 Different winds speeds may be used based on the actual location, however typically 32 kph (20 mph) is
used for flame radiation. Design wind speed should be based on metrological data for the actual
installations location.
 For fixed platform design, consider the worst case for wind direction.
 Floating facilities that are turret mounted may need reduced considerations for wind direction, i.e. where
vessel can face into the wind reducing impact on living quarters and work areas.
f)
Living quarters, drilling deck and helideck:
 Generally the flare is located to obtain the maximum separation distance from the living quarters, drilling
deck and helideck. Considerations may include flame radiation, gas/liquid dispersion and thermal plume.
g) Cranage (cabin), derrick or monkey board (working platform along the side of the drilling derrick):
 These are locations where people may be working at elevated heights.
h) Temperature:
 If flaring events can continue for an extended period of time, metal surface temperatures due to radiant
heat can be a significant design constraint.
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i)
Emergency egress:
 Routes for safe evacuation.
D.1.2 Structural Type Considerations
FO
For offshore applications there are three main flare structure types which should be considered:
 vertical flare stack (platform mounted);
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 boom flare (platform mounted), i.e. cantilevered boom mounted at an angle to the horizontal; and
 vertical flare stack mounted on a separate flare structure connected to main platform by a bridge.
In addition to the above, hybrid solutions which use a combination of vertical and cantilevered boom sections are
sometimes used.
When selecting the flare structure type the following should be considered.
a) Vertical stacks:
 Type of platform, i.e. structural loading on platform.
 Radiation levels must be kept to acceptable levels in any wind conditions (not necessarily the highest
wind speed) by appropriate choice of stack height and flare-burner diameter.
 If liquid carry-over occurs, there is a possibility that burning liquids may fall on to the platform below.

The effect of both radiation and hot plume on helicopter operations.
b) Boom flares:
 Type of platform, i.e. structural loading on platform.
 In the event of liquid carry-over there is less danger of burning liquid falling on to the platform.
 A cantilevered flare boom does not utilize platform space, but adds significant structural loads.
 A single lift installation of boom structure is generally feasible during construction and maintenance.
 The angle of the flare-burner and boom may be different with both used to locate the flame further away
from the platform, reducing heat radiation.
 Floating platforms or FPSOs have to be evaluated for pitch and roll associated with ship or platform
movement.
c) Separate flare structure with a bridge or subsea connection to the main platform:
 Heat radiation levels and safety concerns associated with liquid carry over and the effects on helicopter
operations can be reduced with this option.
NOTE
An advantage of this system is that additional equipment may be accommodated remote from the main platform.
 Subsea connection:
 Mechanical integrity of the subsea piping.
 Liquid collection in subsea pipelines between platform and flare-burner – risk of liquid ejection from
flare-burner.
 Pilot ignition if subsea pipelines are used.
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 Difficult to design for flame-front generators (buildup of condensate at low point).

High-voltage subsea cabling if electronically ignited pilots are used.
D.1.3 Height Considerations
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Flare stack height (equivalent in length to a boom flare) is usually set by heat radiation, but the following factors
should also be considered.
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 In the event of the flare being extinguished, gas dispersion back to the platform and to helicopter flight paths.
 The maximum acceptable concentration of any toxic substance at occupied locations.
 The hot plume temperature levels on the platform and on helicopter flight paths.
 Noise.
 Access for maintenance.
 Associated weight, constructability and maintainability.
D.1.4 Thermal Radiation Shielding Considerations
Radiation shielding may be used at critical locations to protect personnel and equipment against excessive heat
radiation to comply with industry best practices. Aspects to consider are covered in API Standard 521, Table 5.
There are two commonly used methods for shielding against radiation.
a) Barriers:
 These are physical obstacles placed in front of the protected area or object. Barrier may be total (e.g. a
wall) or partial (e.g. a louver or a mesh). Barrier can provide permanent or temporary shielding. Typical
materials used include glass fiber blanket for walls and stainless steel type 310/316L wire mesh for partial
barriers.
b) Absorption:
 Typically for intermittent use, i.e. water spray systems.
Mesh heat shields have more open area (typically 25 % more) as compared to the louvered type, which enables
personnel to see through the shield while being protected from the radiation. Use of mesh type radiation shielding
can have structural design benefits, i.e. reduction in dead weight and wind loading.
For Production platforms, the use of water spray shields during high rates of continuous flaring are not
recommended as the primary means of reducing heat radiation. Water spray systems cannot be relied upon to
reduce peak blow-down radiation loads to tolerable levels since they are inherently unreliable in emergency
situations. There is also the important problem of salt water corrosion to consider.
For drilling platforms, water spray shields may be used for a manual well test where the flaring can be shutdown
readily.
Specific items of equipment for which additional radiation shielding may be considered include:
 crown block bearings;
 sheaves on the drilling derrick;
 critical equipment (e.g. process facilities, flammable storage, critical instrumentation, etc.).
NOTE 1
Critical equipment should be located away from high radiation if not covered by initial design.
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the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
NOTE 2
With angled boom mounted flares, some shielding near the extremity of the boom is required if the flare-burner is
expected to produce higher than normal heat.
Areas where additional protection may be considered for personnel are:
FO
 drilling derrick locations, such as the crown block level and monkey board, including access ladders;
 any areas requiring frequent and prolonged access under all weather conditions such as pathways to
equipment that require access during flaring events;
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 sheltered area designed for heat radiation.
NOTE
Personnel protection should be considered for pathways to protect against high radiation zones if this has not
been addressed in the original design.
D.1.5 Structural Considerations
The user needs to provide to the structural designer any static and dynamic load considerations specific to
offshore installations, structures and transportation.
Platform mounted flare stacks and flare booms should comply with applicable codes to withstand all dead and live
in-place loads such as the following.
 Dead loads that comprise self-weight of the structure and equipment thereon.
 Live loads that include wind loads, jet thrust loads at the flare-burner, operational loads (including pulsation or
surging resulting from the process system) and thermal/expansion loads which may be particularly severe at
the end of the structure, adjacent to the flare-burner.
 Static and dynamic (fatigue), platform motion/acceleration conditions.
 Special loads such as ice buildup, loads imposed during transportation, etc.
 The structural designer should ensure that the frequency of the flare structure is not excited by vibrations
resulting from environmental loads on the platform sub-structure, module support frame, or from process
induced vibrations. Furthermore, individual member vibrations from wind induced vortex shedding should be
avoided.
D.2 Design Considerations for Flare-burners
D.2.1 General Considerations
An offshore flare-burner design should include additional considerations over normal flare design including:
 longevity, because of the unique access and availability constraints;
 reduce radiation, because of proximity to structures and accommodation;
 reduce or eliminate smoke formation;
 allow a high gas exit velocity compatible with stable, efficient combustion;
 perform safely and reliably in emergency conditions and therefore, not be subject to blockage or being reliant
on services that may not be available in an emergency;
 minimize noise;
 minimize maintenance requirements;
 improved liquid handling capability.
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the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
A fundamental decision to be taken regarding flare-burner selection is whether or not a high-pressure (sonic)
flare-burner is required. Although high-pressure flare-burners are used on most offshore installations, they are not
suitable for all discharge sources and consequently, a low-pressure (conventional) burner is generally used in
parallel.
FO
The suppression of smoke is achieved by reducing the release of elemental carbon from the flare flame. A
common method of doing this in onshore facilities is with steam injection. For offshore systems, it is impractical to
consider steam as a means of smoke suppression and no further discussion of its use will be considered here.
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The use of sonic flares allows for smokeless flaring without the need for a smoke suppression medium (air/steam).
It is more difficult to design for smokeless low-pressure flare-burners since steam is not generally available and air
assist may not be practical.
Gas assist may be used on a low-pressure flares.
Also see A.1.7 Endothermic (fuel-gas assisted) flares.
For high-pressure flare systems, see Section A.2.6 considerations.
D.2.2 Firing Angle
Flare-burners can be installed at any angle from vertically to horizontal.
Vertically fired flare-burners have been found to result in increased longevity as compared to angled flare-burners,
while a 30° to 60° angled burner reduces radiation and noise levels. Angled flare-burners also project any
inadvertent liquid flaring away from the source.
Additional considerations for angled flare-burners include:
 direction of thrust loads relative to the flare boom/header,
 purge rates (higher compared to vertical flare-burners) and
 pilot stability (less wind resistant, may require additional windshields).
D.2.3 Materials of Construction
Also see 5.4.
Grade 304 SS/304L SS should not be specified for offshore applications becuase of corrosion due to the salt
environment.
The materials of construction for flare-burners should be selected considering both material thickness and
specification which are suitable for use in the high temperatures prevailing at the burner level. For burners, heatresistant stainless steels, such as type 310 and/or type 316L, are normally specified as a minimum. Materials,
such as RA330, 800H or Inconel 625, are typically chosen for overall life cycle cost (see D.6) for more severe
applications, since they have better thermal and corrosion properties, as compared to type 310 SS or type
316/316L SS.
Thermal cycling is typically found to be a limiting factor in flare-burner tip life. Flame-lick and low flows are also
issues. Also see A.2.7.4.
Minimize welds in the upper section of the flare-burner exposed to possible flame impingement.
Vendors should always be consulted regarding material selection and their proposals should be discussed with
materials/corrosion specialists.
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the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
Typical materials used for stagnant sea water systems are Monel 8 /cupro-nickel for piping and
Hastelloy9/brass/bronze for water nozzles. When sea water piping is flushed with fresh water immediately after
use, conventional materials, such as type 310 SS or type 316L SS, may be used.
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D.3 Flare Pilots, Ignition and Purging Systems
D.3.1 Flare Pilots
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See Section 4 and Annex B
D.3.2 Flare Pilot Ignition System
The standard ignition systems available and commonly used to reliably ignite pilots include:
 electronic pilot ignition systems,
 spark ignition of a pilot gas/air mix and
 flame-front generators.
Redundant pilot ignition systems may be considered for enhanced availability and may use different technologies.
Historically, flame-front generator type pilot ignition systems have been provided for offshore flare systems.
However, significant reliability/operability problems have been encountered with these systems if used for with
subsea lines. Where they are currently used, the design, maintenance and operation should ensure:
 reliable ignition under all expected weather conditions,
 reliable and consistent supply of and dry fuel gas and air and
 capability to drain ignition lines (particularly if they are subsea).
D.3.3 Flare-burner Direct Ignition
Direct ignition of a flare-burner is sometimes used in offshore applications. In applications such as this, the
flare-burner is not equipped with a pilot burner and flare gas is directly ignited at the outlet of the flare-burner. The
ignition system may be activated either continuously or when the flare gas is sent to the flare-burner.
Direct ignition may be used either in full manual or in an automatic configuration.
Use of direct ignition creates additional hazards and considerations compared to traditional flare system with
continuous pilot(s).
Potential differences that need to be evaluated include:
a) Hazards
8 Monel® is a registered trade name on INCO family of companies. This is an example of a suitable product available
commercially. This information is given for the convenience of users of this part of API Standard 537 and does not constitute
an endorsement by API of this product.
9 Hastelloy® is a registered trade name on INCO family of companies. This is an example of a suitable product available
commercially. This information is given for the convenience of users of this part of API Standard 537 and does not constitute
an endorsement by API of this product.
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the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
1) On a flare, such as that used for onshore facilities, the pilot is ignited before plant start-up. Any pilot
ignition problem can be fixed before relief gas is potentially released to the flare system. For direct ignition,
the ignition system has to be available upon request.
FO
2) If the flare is being purged with inert gas when the direct ignition system is activated, the flare-burner may
be surrounded by inert gas that may compromise the ignition. An unacceptable amount of unburned relief
gas may be released to the atmosphere while an ignition attempt is completed.
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3) A flare-burner main flame is less stable than a pilot flame. Flare-burner main flame extinguishment
probability is increased, especially when flow rate and/or flare gas composition to the flare is not stable.
4) For an automatic or manual configuration of an ignition system on a staged flare, a safety device, such as
a rupture disk or buckle pin valve, is typically installed in parallel to the automatic flare valve. In case of an
undetected opening of such devices, or in case of leak through automatic flare valve or buckle pin valve,
combustible gas will be released unburned to the atmosphere. Also see A.11.3.
b) Considerations
1) Combustible gas should not be used downstream of the flare valve as a purge gas medium in normal
operation, in order to avoid release of unburnt combustible gas to the atmosphere. Dry inert purge gas
should be used. In case of full manual configuration, flare header may be purged from flare header
isolation valve to flare-burner just prior to flaring operation. This only applies to systems with flare gas
recovery or other valve isolation of the flare header.
2) Traditional flare safety is ensured by proven pilot flame. It is more difficult to prove a flare-burner main
flame than a pilot flame since proven detection methods are available to confirm pilot flame.
3) Although environmental regulatory requirements may require the use of continuous pilots, direct ignition
methods may be deemed acceptable to the regulatory authorities based on field testing which
demonstrates successful ignition of the flare-burner under operating and environmental conditions such
as would be expected for ignition with a continuous pilot.
c) Various means of flare direct ignition used include the following.
1) Pellet ignition: an air gun device located at flare base fires a pellet to a plate located adjacent to the flareburner. The pellet release sparks around the flare-burner.
2) Direct electronic ignition: the system is similar to the one used for pilot. Spark is created by electrode at
outlet of flare-burner. The system is much more exposed to flame and temperature than in a pilot and may
require frequent maintenance or replacement.
3) Manual gun ignition: a signal flare gun or equivalent is usually used in manual configuration, and
frequently considered as a backup ignition means. However, the manual gun ignition technique has to be
a controlled operation due to the danger associated with carrying such a flare gun around a production
installation.
D.3.4 Purge Gas
Purge gas requirements for offshore flares are consistent with those for elevated flares with special considerations
to be given to firing angle, where applicable.
D.4 Maintenance and Inspection Considerations
The following are the various methods of inspection for offshore flares:
 unmanned aerial vehicle (UAV) with camera,
 telescope (binocular),
 manned helicopter,
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the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
 weather balloon with camera, and
 physical inspection by rope access/scaffold (flare off-line).
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This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
The following are the various means of access for repairs of offshore flares:
 helicopter,
 lift boat,
FO
 crane, and
 davit/gin pole.
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D.5 Flare-burner Inspection Frequency
The frequency of inspection varies due location and service. Indicative values are given below.
 1 to 2 year formal remote visual of tip and ancillaries of operation.
 3 to 5 year up-close inspection of tip and ancillaries.
extended pending initial inspection findings.
This requires shutdown, and frequency may be
 Anytime the flare is down and isolated, a close inspection may be considered.
 After extended emergency operation or appearance of abnormal performance carry out a remote visual
inspection.
D.6 Considerations for Offshore Applications
D.6.1 Vertical Flare Stacks
A vertical stack may utilize a cantilevered support structure, from the side of a module, or module support frame to
avoid occupying valuable platform space.
D.6.2 Flare Boom Stacks
The angle of the boom flare affects the lifetime of the flare-burner.
D.6.3 Separate Flare Structure with a Bridge or Subsea Connection to the Main Platform
This option requires a separate jacket structure and bridge. The final choice of structure type will require a
detailed investigation of the relative merits of the options available for the particular application under
consideration.
D.6.4 Structural
The flare structure adjacent to the flare-burner may be subject to thermal gradients resulting in high stresses.
Under these circumstances it may be necessary to specify special steels, e.g. stainless steel. Alternatively, to the
user may use a heat shield and/or protective paints/passive fire protection for critical members. An engineering
analysis should be considered to aid selection of the correct approach to use.
D.6.5 Materials of Construction
In offshore applications, the high working temperatures are combined with corrosion problems due to sea water
attack. Maintenance and replacement of the burner on the platform is difficult. For this reason, it may make sense
to select materials and a design which give the best possible life.
D.7 Reliability Analysis
Several reliability and probability analysis methods exist and may be used to evaluate the design performance of
any operating flare.
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Annex E
(informative)
Instructions for Flare Data Sheets
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E.1 Introduction
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This annex includes instructions for completing API 537 flare data sheets (see Annex F).
These data sheets are designed to provide a concise, but thorough, definition of the flare system and its
performance. The data sheets should evolve throughout the course of a project. The level of detail reflected in the
data sheets should be consistent with the current stage of the project. Early in a project, the sheets may contain
less detail than later revisions. Some of the fields on these sheets may remain blank if the information is not
known or not relevant to the particular application. Users of these data sheets are encouraged to apply
reasonable judgment in determining which fields apply.
It is intended that these data sheets become the controlling document in specifying flare equipment. It is important
that all parties involved with the flare, including vendors, engineering contractors, purchasers and end users share
a clear understanding of the meaning of each field. While many of the fields are self-explanatory, some require
clarification beyond the wording of the field labels. These instructions describe in more detail fields whose labels
can be inadequate to fully define their purpose. In addition, to support the goal of defining the flare system, it is
often appropriate to append a process control diagram to the data sheets at the start of a flare project.
Data sheets are divided into groups to facilitate use. Data sheets designated as Form General 1 to
Form General 7 [F.1 a) for SI units and F.2 a) for USC units] set forth the general information regarding a project
and may be used for any type of flare, elevated, enclosed, etc. Information specific to an elevated flare can be
recorded on the data sheets that are designated as Form Elevated 1 to Form Elevated 5 [E.1 b) for SI units and
E.2 b) for USC units]. Enclosed-flare data belongs on sheets designated as Form Enclosed 1 to Form Enclosed 5
[E.1 c) for SI units and E.2 c) for USC units]. Thus, a combination of “General” and “Elevated” forms can be used
to specify an elevated-flare system.
These data sheets cover both mechanical and process aspects of flare design. Those using the data sheets are
referred to API Standard 521 for process information. The combination of API Standard 521 and this standard
provides a broad source of information for those interested in flares.
All forms have a line in the header at the top that contains Page ____ of ____. This page numbering system is an
integral part of the “General Notes” system. The preparer of this form is strongly encouraged to include both page
numbers and total pages on all forms. In the event that subsequent revisions result in additional pages (such as
additional gas stream or notes pages), it is recommended to modify the page numbers by using 3A, 3B, etc. for
gas stream pages, as an example. This avoids having to renumber all pages and note references on Form(s)
General 7 and prevents the confusion that can result from renumbering errors. Changes to the total page count at
the top of each page are necessary whenever pages are added to the package.
All forms have a column labeled “Note”, which is intended to refer to additional notes on one or more copies of
Form General 7: General notes. Numbering of the notes should start with one (1) on each new page. The liberal
use of explanatory notes is strongly encouraged to ensure a clear communication of all job requirements.
EXAMPLE:
A system using a Form General 3 to define the flow conditions can be more clearly described by placing a
numeral “1” in the N column on line 1 or 2 even though previous pages already contain notes. On a copy of Form General 7,
the user would place a note referring, perhaps, to Page 3, Note 1. The note can define normal flow rates, frequency and
duration for various streams on Form General 3, as opposed to maximum hydraulic flow or smokeless capacity required. Such
a note helps both the designer and the operator to understand how the equipment will actually be used.
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the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
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It is to be expected that revisions will occur to the data sheets during the course of a project. All forms include one
or more columns labeled “REV” where a revision can be marked. In addition, the heading section of each form
contains a “revision number” field. When a set of changes is made to a set of data sheets, this set of changes is
referred to as “a revision” and is assigned the next revision number. The original issue should be noted as
“revision zero (0).” All changes made in a revision are marked with the same revision number. As a matter of
reference, a copy of Form General 7 containing the revision history should be included. Each revision note should
contain, as a minimum, the revision number, the revision date and some description of the revision such as
“Revised per Vendor Quote” or “Revised for Purchase.” Additional information, such as a list of affected
forms/lines, can be useful for tracking purposes. Each revision should be issued as a complete set of pages, not
as individual pages. This ensures that all recipients have a complete, current set of data sheets.
E.2 General Information Forms — Instructions
E.2.1 Form General 1
Form General 1
Line 9
Jobsite climate
Indicate type of climate such as dusty desert, arctic tundra or tropical jungle.
Can indicate a requirement for dust filters, freeze protection, special
radiation considerations, instrument packaging, etc.
Line 16
Local codes
State or local codes can affect electrical equipment, mechanical design,
process performance, shipping or other aspects of a major construction
project such as a flare system. Any such regulations that can affect the
design, fabrication, delivery, construction or operation of the system should
be identified as early in the project as possible.
Line 19
Ambient
conditions
(design/normal)
Each of the conditions listed has design values and normal values. Design
values can be necessary for proper selection of metallurgy or piping growth.
Normal values can provide a better idea of conditions that will normally
prevail and can allow for certain operational efficiencies most of the time.
Provide minimum and maximum temperatures as they influence items such
as blower design, structural materials, or thermal growth/shrinkage.
Line 22
Relative humidity Some radiation models allow a credit for atmospheric attenuation at large
distances. Atmospheric humidity can affect smokeless performance,
electrical circuit design, etc.
Line 26
Predominant
wind direction
If the jobsite has a very predominant wind direction, it is sometimes possible
to design the system to take this into consideration. A wind rose can be
provided if it is available. Suitable orientation of pilots, for example, can allow
longer equipment life by avoiding the predominant flame pull-down area.
Lines
27 and 28
Solar radiation
Refer to API Standard 521 for a discussion of solar radiation allowances.
Line 29
Jobsite elevation
Altitude of jobsite affects local atmospheric pressure, which affects pressure
drop calculations, fan sizing, etc.
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the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
E.2.2 Form General 2
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Form General 2
Line 1
Minimum flare
height
Nearby structures, electrical classification issues, independent dispersion
calculations or company standards can impose a minimum flare height
requirement.
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Line 2
Anticipated flareheader diameter
Fields in lines 2, 3 and 4 allow the designer to estimate the flare-header
volume, surface area, pressure drop, etc. These factors can affect purge
system design, peak waste-gas flow rate or actual gas temperature arriving
at the flare, and other important design issues. It is sometimes possible to
anticipate transient behaviors in the flare system that can affect overall
performance. Flare-header volume includes all piping and drums that can be
pressurized by a flare event regardless of whether the relief actually passes
through that section of the flare-header system.
Line 3
Approx. flareheader length
Line 4
Flare-header
network volume
Line 5
Plot space
available
This can affect selection of the support method, size of component parts,
guy-wire radius, etc.
Line 11
Special erection
requirements
Plans to construct a system using gin poles in lieu of a crane, single-point
lifting requirements, limited lay-down areas for construction or preference for
bolted construction are examples of special requirements that it is necessary
to define early in a flare project.
Lines
13 to 16
Nozzle location
and loads
The position of the relief-gas nozzle connection to the gas riser is of primary
importance to the structural and foundation design of the flare. The minimum
elevation is typically imposed by the presence of a liquid seal and/or
knockout drum. The nozzle location, unless otherwise specified, shall be
10 m (328 ft). Refer to Table 2 for allowable nozzle forces and moments.
Line 24
Utilities available
(design/normal)
Each of the conditions listed has design values and normal values. Design
values can be necessary for proper selection of metallurgy or piping growth,
for example. Normal values can provide a better idea of conditions that will
normally prevail and can allow for certain operational efficiencies most of the
time.
Line 27
Location of steam Steam temperature and pressure vary from one point in the steam system to
conditions
another due to heat losses and pressure drops. It is necessary that the
designer know whether the indicated pressure is available at the flareburner, at the base of the stack, at a point outside a sterile radius or at a
boiler somewhere. It is necessary that the designer also know whether the
pressure and temperature are downstream or upstream of the control valve.
Lines
28 and 29
Electrical power
It is important to know whether the local power supply is 50 Hz or 60 Hz, as
this has a profound effect on blower-motor performance. It is necessary that
the voltage be known before vendors can select appropriate control
equipment.
Line 34
Fuel gas
One of the compositions that should be defined on a copy of Form General 3
is that of the fuel gas to be used for pilots, flame-front generator, enrichment
gas, etc. As a minimum, it is necessary that the designer know the MW and
LHV of the fuel gas. If the fuel gas contains more than 10 % volume fraction
hydrogen, unsaturated hydrocarbons, hydrogen sulfide or inerts, then it is
necessary that the composition be identified.
Line 35
Purge gas
Purge-gas composition should be defined on a copy of Form General 3.
Line 38
Nearby structures Flares are usually sized to meet a specified radiation criterion at grade.
(distance, height) Radiation on nearby structures, especially heat-sensitive structures, such as
cooling towers, can be accounted for only if such structures are identified
and located.
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Lines
39 to 42
Other active
flares
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If there are other flares in the vicinity of the specified flare that are expected
to be flaring simultaneously with the specified flare, these should be
accounted for in the design of the specified flare. In order to account for such
flares properly, some clear definition of the other flare’s radiation information
is necessary. Heat release and radiant fraction, as a minimum enable only a
rough accounting. Direct information, e.g. isopleths from the other flare’s
vendor, is preferred. It should be included by reference with a note and any
attachments that can be useful. Consideration can be given to doing
maintenance work on one flare while any nearby flare is operating.
E.2.3 Form General 3
Form General 3
Line 2
Smokeless
capacity, opacity
Smokeless capacity is defined on the data sheets in kg/h (lb/h), rather than
some percentage of design flow. The smokeless-capacity requirement
should be established by a thoughtful review of actual relief scenarios.
Conditions that are expected to occur often enough to require smokeless
operation, either by regulation or company standards, should set the
smokeless requirement.
Indicate the opacity, or Ringleman number, that is allowable at the flow rate
for smokeless operation.
Line 4
Static pressure
Static pressure, in this context, is the pressure exerted by the gas on the
walls of the flare header. This pressure determines the gas density. A
conventional pressure gauge mounted on the side of a pipe measures static
pressure. An additional component of pressure at the flare inlet is the
velocity pressure.
The sum of these two components is called “total pressure,” also known as
“stagnation pressure.” The total pressure is a good measure of the energy
available in the flowing fluid. A properly positioned pitot tube measures the
total pressure on the port facing the flow stream. Due to the tendency for
plugging, pitot tubes are not often used for common pressure
measurements.
Velocity pressure can be calculated for a given flow stream if the static
pressure and pipe diameters are known. This approach allows the use of
conventional pressure gauges to check performance. This is the reason for
requiring declaration of both static pressure and diameter at the flare inlet. If
the purchaser does not define the flare inlet diameter, then the specified
pressure should be indicated as total pressure.
Pressure is based on relieving conditions as identified by different operations
specified on these data sheets.
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the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
Line 6
Veq
𝑇
𝑉eq = 0.2518 × 𝑞𝑚 × √𝑀gas
(E.1)
gas
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Veq is the volumetric flow rate of air at standard temperature that produces
the same velocity pressure as the actual gas flow at the same compression
and actual gas temperature. It is proportional to the waste-gas flow rate and
is independent of the pipe diameter used to evaluate velocity pressure. The
volumetric flow rate, Veq, is given by Equations E.1 to E.4:
𝑉eq = 𝛿normal × √
where
Veq
qm
Tgas
Mgas
 normal
𝑀gas
29
𝑇gas
× 273
(E.2)
3
is the volumetric flow rate of air (Nm /hr) at 0 °C;
is the mass flow rate of gas, expressed in kilograms per hour;
is the absolute temperature of the gas, expressed in degrees
Kelvin;
is the relative molecular mass of the gas;
is the volumetric flow rate, expressed in normal cubic meters
per hour at 0 °C.
𝑇
𝑉eq = 3091 × 𝑞𝑚 × √𝑀gas
gas
𝑀gas
𝑉eq = 𝛿SCFH × √
29
×
𝑇gas
520
(E.3)
(E.4)
where
Veq
qm
Tgas
is the volumetric flow rate of air (SCFH) at 60 °F;
is the mass flow rate of gas, expressed in pounds per hour;
is the absolute temperature of the gas, expressed in degrees
Rankine;
Mgas
is the relative molecular mass of the gas;
 SCFH is the volumetric flow rate, expressed in standard cubic feet per
hour at 60 °F.
It should be noted that Veq is intended as a means to compare hydraulic
performance or requirements among flowing conditions at a fixed jobsite. If
comparisons to other jobsites at other altitudes are required, then a
correction is required for atmospheric-pressure variations.
Line 8
Duration at max.
rate
Duration of the relief can affect allowable radiation levels, noise levels,
smokeless requirements and other aspects of the design.
Line 9
Relief source
Some indication of the relief source and its cause is useful to the designer.
A label, such as “Power failure” or “Demethanizer overheads,” can help both
for communication about cases and for understanding the character of the
relief.
Line 10
Controlling case
for
Indicate whether this relief case is the controlling case for pressure drop
(DP), radiation (RAD), noise (NOI), smokeless performance (SMK), etc.
Line 11
Gas composition
It is necessary that the designer know whether the specified composition is
on a mass or a molar basis to properly evaluate stream properties. Either
circle one of the options (if that option applies to all streams) or define the
basis explicitly for each stream.
Lines
12 to 37
Compounds
A blank line has been left at the end of the list to allow for inclusion of an
additional compound not found on the pre-printed list. If necessary, one or
more of the unused compounds in the pre-printed list can be struck through
and replaced with additional unlisted compounds.
119
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the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
Total
Ideally, gas composition should total 100 %. Compositions are sometimes
provided in the form of flow rates of each component, in which case the total
of flow rates should match the design flow condition.
Lines
39 to 45
Hydrocarbon
characterization
information
This information is used for combustion, smoking tendency and hydraulic
considerations.
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E.2.4 Form General 4
Form General 4
Line 8
Flame monitors
Indicate the number of flame monitors required and whether this count is per
pilot or per flare.
Line 9
Flame monitor
type
Indicate type K (or other) thermocouples, optical, ionization, acoustic
detectors or as appropriate.
Line 14
Retractable pilots This information is used primarily for enclosed flares. Indicate whether pilots
should be removable while the flare is in service.
Line 15
Retractable
thermocouples
Indicate whether pilot thermocouples should be removable while the flare is
in service.
Line 21
Distance from
stack
Indicate the distance in terms of piping length from the ignition panel to the
flare stack. This can be substantially longer than simple radial distance if the
piping runs along a pipe rack.
E.2.5 Form General 5
Form General 5
Line 4
Integral/separate
from stack
Indicate whether it is required that this vessel be integral with the stack or
separate from the stack. If the vessel is built into the base of an elevated
structure, high corrosion rates or a requirement to bypass and isolate the
vessel while the flare is in service can require a separate vessel.
Line 10
Seal depth
Seal depth determines the inlet pressure at which the first bubble of gas
flows through the vessel. Design seal depth varies depending on the
purpose of the liquid seal. Simple maintenance of a positive upstream
header pressure can require only a few centimeters of depth. Flare-gas
recovery systems often require 500 mm to 750 mm (20 in. to 30 in.) of seal
depth to ensure adequate suction pressure for the compressor. Liquid seals
used for staging between multiple flares can have seal depths of 2.5 m
(100 in.) or more.
Line 11
Max. vacuum
Flare-gas recovery systems or hot-gas thermal contraction and/or
condensation can result in substantial vacuums in the flare header. A vertical
section of piping in the liquid-seal inlet line can allow seal fluid to be drawn
up by the vacuum without drawing air in through the flare-burner. This
protects the plant against a potentially dangerous situation. To achieve this
level of protection, it is necessary to design the vessel with sufficient liquid
volume in the normal seal-depth area to fill the vertical section of piping.
Safe design of this liquid volume should take no credit for the addition of
supplemental liquid. Operationally, it is necessary to maintain the proper
liquid level in the liquid seal and to restore that level promptly after any hot
relief and before the vacuum forms. The maximum vacuum protection
achievable can be limited by piping or vessel elevations.
120
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the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
Lines 13
and
17 to 29
Various
connections
Each of these lines asks for a description of a vessel connection, including
the type of connection (flanged, threaded, welded), the size in millimeters
(inches) and the number of these connections.
FO
E.2.6 Form General 6
Form General 6
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Integral/separate
from stack
Indicate whether it is required that this vessel be integral with the stack or
separate from the stack. If the vessel is built into the base of an elevated
structure, high corrosion rates or a requirement to bypass and isolate the
vessel while the flare is in service can require a separate vessel.
Line 6
Design code
The purchaser shall specify.
Line 11
Max. liquid level
Maximum liquid level may be defined either as a distance above bottom
tangent or as an absolute elevation. The vendor may define this value to
prevent re-entrainment of accumulated liquid in the waste-gas stream.
Line 12
Liquid holdup
volume
The purchaser, based on the anticipated liquid volumes that can be sent to
the flare system, may define the liquid-holdup volume. Sufficient volume
should be provided to prevent overfilling of the knockout drum, which can
lead to liquid carryover to the flare-burner, smoke, flaming rain and other
hazardous conditions.
Lines 14
and
18 to 30
Various
connections
Each of these lines asks for the description of a vessel connection including
the type of connection (flanged, threaded, welded), the size in millimeters
(inches) and the number of these connections.
E.2.7 Form General 7
The “Page No.” and “Note No.” columns are intended to allow all notes associated with all pages to be collected
on a single set of pages appended to the back of the data sheet package. “Page” and “note” numbers should
precede each note to indicate the location in the data sheet package to which the note refers. Notes can be
several lines long and require the “page” and “note” references only on the first line.
E.3 Elevated-flare Forms — Instructions
E.3.1 Form Elevated 1
Form Elevated 1
Radiation and noise performance is often specified in terms of the maximum flaring rate. Similarly,
smokeless-performance specifications require smokeless operation up to some specified flow rate. In
practice, it is often the performance of the flare at rates substantially below maximum and below peak
smokeless capacities that actually determines whether the flare is acceptable to the user or the community.
Some representation of these turndown conditions can be provided as an additional gas stream on Form
General 3. Performance expectations for these conditions can be specified either by using one of the blank
lines on Form Elevated 1 or through the use of General notes.
Line 15
SPL at flare base Unless otherwise specified, noise at the base of the flare is defined at a point
1.5 m (5 ft) above grade and 10 % of the flare stack height distance from the
flare stack centerline. Nearby noise sources, such as blowers or steamcontrol valves, should be identified and a general note should indicate
whether or not these nearby sources are included in the noise prediction.
Lines
16 and 17
SPL at distance
Noise at a distance is measured 1.5 m (5 ft) above grade at the specified
distance from the flare stack centerline. If typical background noise levels in
the target area are known, they should be indicated with a general note.
121
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publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
Line 21
Smokeless
definition
Environmental regulations usually specify that a flare may not exceed some
opacity level for more than a certain amount of time. That opacity level
defines the smokeless criteria. An opacity level of 20 % corresponds to
Ringleman 1.40 % to Ringleman 2. Zero opacity is Ringleman 0.
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E.3.2 Form Elevated 2
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Form Elevated 2
In some cases, it can be necessary to add a general-notes page to include any clarifications in the areas of
predicted system performance.
Line 2
Static inlet
pressure
Normally, this should be based on the specified flare inlet diameter (Form
General 3, Line 5). If the vendor is proposing a different flare inlet diameter,
the proposed diameter should be clearly defined on this line such as
“20 kPa @ 600 mm” (“3 psig @ 24 in. inlet”).
Line 11
(blank)
Some vendors provide radiation information in the form of graphical outputs.
Such outputs should be appended to the data sheets and may be referred to
here by filling in “Radiation plot” as the description and “See attached” as the
value.
Lines
13 to 17
Noise
performance
Some vendors provide noise information in the form of tables showing
octave band breakdowns. These can be appended or included as general
notes.
Line 27
S/HC ratio
Steam consumption has often been characterized in terms of mass ratios of
steam to hydrocarbon required for smokeless performance. The value
provided on this form is based on operation at or near the smokeless
capacity. The purchaser is cautioned that the ratio necessary for waste-gas
flows in the turndown range can exceed the ratio near the smokeless
capacity.
Lines
32 and 33
Air capacity and
pressure
The vendor should clearly indicate whether the pressure basis is static or
total pressure at the blower outlet. If static pressure is used, the outlet area
of the fan should be indicated.
E.3.3 Form Elevated 4
Form Elevated 4
Line 8
Loop-seal depth
Some purge-conservation devices, such as buoyancy seals, include a drain
to continuously remove rainfall, steam condensate or other liquids that can
enter the seal. A loop seal, similar to that used for an API knockout drum or
liquid-seal skimmer, should be used to prevent flare gas from migrating into
the drainage system. Refer to API Standard 521 for further information on
determining this depth. The required depth of this loop seal should be
defined on this line.
Line 12
Stack design
pressure
The purchaser is cautioned against excessively high design pressures as
the combination of stack loadings from wind, earthquake and internal
pressure can result in much thicker walls than are actually required.
122
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publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
Line 13
Stack design
temperature
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When gas temperatures differ substantially from ambient temperature, a
significant heat transfer rate can exist between the waste gas and ambient
air. This heat transfer can affect stack-design temperatures in two ways.
First, heat transfer to or from the waste gas while it is flowing from the plant
to the flare stack generally causes the waste-gas temperature to move
closer to ambient temperature. Second, the steel temperature is somewhere
between the waste-gas temperature arriving at the flare stack and the
ambient temperature. Both of these effects should be considered when
establishing the stack design temperature to avoid over-specification. The
vendor and purchaser can work together to specify this temperature, if so
noted on the data sheet.
E.3.4 Form Elevated 5
Form Elevated 5
Line 10
Max. motor
current – winter
As discussed in A.9.3, flare fans deliver a certain maximum volumetric flow
of ambient air to the flare-burner. At minimum ambient temperatures, the
density of this air is the highest. As a result, the motor horsepower required
is highest in winter. The electrical current required to drive the motor under
these conditions usually dominates the design requirements for the
switchgear and substation that delivers this power to the fan motor.
E.4 Enclosed-flare Forms — Instructions
E.4.1 Form Enclosed 1
Form Enclosed 1
Line 1
Enclosed
capacity
Indicate the maximum continuous flow rate that the enclosed flare shall
handle without visible flame, excessive temperature or noise.
Line 7
SPL at wind
fence
Noise at the wind fence is measured 1.5 m (5 ft) above grade at a distance
of 0.9 m (3 ft) from the major bounding surface at the base of the flare. This
is usually the wind fence. Nearby noise sources, such as blowers or steamcontrol valves, should be identified and a general note should indicate
whether or not these nearby sources are included in the noise prediction.
Lines
8 and 9
SPL at distance
Noise at a distance is measured 1.5 m (5 ft) above grade at the specified
distance from the flare stack centerline. If typical background noise levels in
the target area are known, they should be indicated with a general note.
Lines
27 and 28
Purge gas
Staging is often used in enclosed flares to improve turndown performance.
A continuous purge is recommended to keep the flare header swept clear
and to prevent air ingression through the first stage of burners. In many
cases, a brief, relatively high purge flow is injected downstream of each
staging valve to flush out residual waste gases after that staging valve is
closed. If there are purge-gas-capacity limitations, these should be specified
by the purchaser.
123
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publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
E.4.2 Form Enclosed 2
Form Enclosed 2
Line 1
The vendor should indicate maximum enclosed capacity for the specified
composition from Form Enclosed 1. If there are multiple flow streams, the
vendor should indicate maximum enclosed capacity for each stream as a
general note. Any discussion relating to the interpretation of enclosed flaring
should be included as a general note.
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Enclosed
capacity
Lines
7 to 11
Noise
performance
Some vendors provide noise information in the form of tables showing
octave band breakdowns. These can be appended or included as general
notes.
Line 12
Smokeless
capacity
The vendor should indicate smokeless capacity for the specified composition
from Form Enclosed 1. If there are multiple flow streams, the vendor should
indicate smokeless capacity for each stream as a general note.
Line 19
S/HC ratio
Steam consumption has often been characterized in terms of mass ratios of
steam to hydrocarbon required for smokeless performance. The value
provided on this form is based on operation at or near the smokeless
capacity. The purchaser is cautioned that the ratio necessary for waste-gas
flows in the turndown range can exceed the ratio near the smokeless
capacity.
Lines
27 and 28
Purge gas
The vendor should indicate both the continuous purge requirement and the
maximum intermittent purge flow requirement during staging operations.
Line 33
Supplemental
gas
If supplemental fuel gas is used to maintain a minimum temperature in the
firebox, the vendor should indicate the flow rate necessary in cold weather.
E.4.3 Form Enclosed 3
Form Enclosed 3
Lines
21 to 29
Firebox and
wind-fence
dimensions
Most enclosed flares fall into one of the following shape categories:
rectangular, round or polygonal. The purchaser should indicate any
preferences regarding shape. The vendor should indicate selected shape
and associated dimensions.
Line 30
Refractory
material
The purchaser should indicate any requirements or limitations on refractory
material.
Line 32
Max. service
temperature
The vendor should indicate service temperature of the proposed refractory.
Line 33
Max. shell
temperature
The vendor should indicate expected shell temperature for ambient
conditions of 27 °C (80 °F) and still air. This calculated temperature is used
to select the paint system for the outside of the firebox. The purchaser
should indicate hot-face temperature basis for calculation as either max.
allowable temperature for the refractory or calculated operating temperature
at the enclosed flaring capacity. Significant cost savings can accrue with the
use of a lower hot-face temperature basis.
Line 35
Max. expected
flue-gas
temperature
The vendor should indicate expected flue-gas temperature for ambient
conditions of 27 °C (80 °F) and still air.
Line 39
Max. personnelexposure
temperature
The vendor should indicate the maximum temperature on any surface where
personnel exposure can occur. This is often limited to the outer wind-fence
surface when access to the upper stack platforms is not necessary during
maximum operation.
124
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publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
E.4.4 Form Enclosed 4
Form Enclosed 4
Line 4
Heat shielding
FO
If air-assisted burners are being used, the purchaser should indicate any
preferences for either a large, single blower with distribution by manifolds
and valves vs. individual blowers for various stages or sections of burners.
The vendor should indicate proposed/actual method for distributing air. Use
general notes, if necessary, to clarify the issue.
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Lines 19,
Air valve
25, 31 and
37
Any material or equipment with a view of the burner windows can be
exposed to high heat radiation. Heat shielding is often used to reduce
metallurgical requirements and piping stresses.
E.4.5 Form Enclosed 5
Form Enclosed 5
Line 5
Damper control
required
The vendor should indicate whether any air dampers/valves are modulated
(based on temperature or flow), automatically opened/closed or manually
set.
Line 11
Supplemental
requirements
The purchaser should indicate the existence of any special requirements,
such as explosion-proof motors or inlet filters, for the air blowers.
125
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publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
Annex F
(informative)
Flare Data Sheets
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The datasheets in this annex are for use by the purchaser and the manufacturer to communicate the system
requirements and/or the manufacturing specification to the interested parties. The interactive Excel10 versions of
these datasheets are available for purchase from authorized API redistributors (www.api.org/publications).
F.1 SI Units
a) General Data Sheets, containing the following 7 sheets:
General 1 — Purchaser Supplied - General Information (Site Specifics)
General 2 — Purchaser Supplied - General Information (Utilities)
General 3 — Process Design Conditions - Purchaser (Flare Gases)
General 4 — Mechanical Design Data (Pilots and Ignition System)
General 5 — Mechanical Design Data (Liquid Seal)
General 6 — Mechanical Design Data (Knockout Drum)
General 7 — Blank for Notes
b) Elevated Flare Data Sheets (5 sheets):
Elevated Flare 1 — Required System Performance - Purchaser
Elevated Flare 2 — Predicted System Performance - Vendor
Elevated Flare 3 — Mechanical Design Data (Flare Burner)
Elevated Flare 4 — Mechanical Design Data (Purge Device/Stack)
Elevated Flare 5 — Mechanical Design Data (Ancillaries)
c) Enclosed Flare Data Sheets (5 sheets):
Enclosed Flare 1 — Required System Performance - Purchaser
Enclosed Flare 2 — Predicted System Performance - Vendor
Enclosed Flare 3 — Mechanical Design Data (Combustion Equipment)
Enclosed Flare 4 — Mechanical Design Data (Piping)
Enclosed Flare 5 — Mechanical Design Data (Ancillaries)
10 Excel® is a registered trade mark of the Microsoft Corporation. This is an example of a suitable product available
commercially. This information is given for the convenience of users of this part of API Standard 537 and does not constitute
an endorsement by API of this product.
126
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the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
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publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
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publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
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publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
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publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
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F.2 USC Units
a) General data sheets, containing the following 7 sheets:
General 1 — Purchaser Supplied - General Information (Site Specifics)
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General 2 — Purchaser Supplied - General Information (Utilities)
General 3 — Process Design Conditions - Purchaser (Flare Gases)
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General 4 — Mechanical Design Data (Pilots and Ignition System)
General 5 — Mechanical Design Data (Liquid Seal)
General 6 — Mechanical Design Data (Knockout Drum)
General 7 — Blank for Notes
b) Elevated Flare Data Sheets (5 sheets):
Elevated Flare 1 — Required System Performance - Purchaser
Elevated Flare 2 — Predicted System Performance - Vendor
Elevated Flare 3 — Mechanical Design Data (Flare Burner)
Elevated Flare 4 — Mechanical Design Data (Purge Device/Stack)
Elevated Flare 5 — Mechanical Design Data (Ancillaries)
c) Enclosed Flare Data Sheets (5 sheets):
Enclosed Flare 1 — Required System Performance - Purchaser
Enclosed Flare 2 — Predicted System Performance - Vendor
Enclosed Flare 3 — Mechanical Design Data (Combustion Equipment)
Enclosed Flare 4 — Mechanical Design Data (Piping)
Enclosed Flare 5 — Mechanical Design Data (Ancillaries)
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Annex G
(informative)
API Standard 521 Sixth Edition Migration
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G.1 General
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This Annex captures informative content migrated from API Standard 521 Sixth Edition through a
API 521 / API 537 joint committee review. Any identified normative requirements contained in API 521 that were
not already included in API 537 have been added the normative section of this standard. The following table
provides cross-reference information for the two standards.
Table G.1-API 521 to API 537 Cross-reference
API 521 Source
API 537 Informative
5.7.2.2
G.2.1
5.7.2.4
G.2.2
5.7.3.2
G.3.1
5.7.3.2.2
G.3.2
5.7.3.2.3
G.3.3
5.7.3.2.4
G.3.4
5.7.3.2.5
G3.5
5.7.3.2.6
G.3.6
5.7.3.2.7
G.3.7
5.7.3.2.8
G.3.8
5.7.3.2.9
G.3.9
5.7.3.2.10
G.3.10
5.7.3.2.11
G.3.11
5.7.3.3
G.3.12
5.7.3.4
G.3.13
5.7.4.1
G.4.1
5.7.4.2
G.4.2
5.7.4.3
G.4.3
5.7.4.4
G.4.4
5.7.5.1
G.5.1
5.7.5.2
G.5.2
5.7.5.3
G.5.3
5.7.5.4
G.5.4
5.7.6.1
G.6.1
5.7.6.2
G.6.2
5.7.7.1
G.7.1
5.7.7.2
G.7.2
5.7.7.3
G.7.3
5.7.7.4
G.7.4
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G.2 Combustion Properties
G.2.1 Smoke (5.7.2.2)
Normative Requirements for Consideration:
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
The range of smokeless operation shall be defined based on the appropriate Ringelmann number for all
operational cases that require smokeless operation.
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G.2.1.1 Introduction
Many hydrocarbon flames are luminous because of incandescent carbon particles formed in the flames. Under
certain conditions, these particles are released from luminous flames as smoke. The exact reasons and mechanisms
by which smoke is formed are still not fully understood. Many different processes have been suggested, but a
discussion of them is beyond the scope of this standard. However, it can be stated that smoke is formed during the
combustion of hydrocarbons only when the system is fuel rich, either overall or locally. Observation has revealed that
[32]
suppression of the hydrogen-atom concentration in the flames accompanies the suppression of smoke formation .
Smoke formation can possibly be reduced by reactions that consume hydrogen atoms or render them ineffective.
G.2.1.2 Design Considerations




Use of water vapour to reduce smoke: The ways in which water vapor reduces smoke from flares have been
[145]
discussed by Smith
. Briefly, one theory suggests that steam separates the hydrocarbon molecules, thereby
minimizing polymerization, and forms oxygen compounds that burn at a reduced rate and temperature that are
not conducive to cracking and polymerization. Another theory claims that water vapor reacts with the carbon
particles to form carbon monoxide, carbon dioxide and hydrogen, thereby removing the carbon before it cools
and forms smoke.
See G.3.1 for a discussion of smoke suppression methods.
Causes of smoke: There are many possible causes for a smoking flare such as liquid carryover, flare gas flow
rate change, change in flare gas composition, or incorrect flow of smoke suppression fluid. Smoking is a visual
signal to check operation (e.g. adjust the flow of smoke suppression fluid). See Annex A, B, C, and D for
operation and troubleshooting guidance related to smoke suppression relative to the various flare types.
Destruction vs. Combustion Efficiency: Although a smoking flare flame is related to combustion efficiency, it is
not directly related to destruction efficiency. The destruction efficiency of the highly branched hydrocarbons can
[60]
exceed 99 %, while simultaneously the residual carbon soot emitted could cause a very opaque plume .
G.2.2 Flame Stability (5.7.2.4)
Normative Requirements for Consideration:
 Flare flame combustion shall be stable over the entire defined operating range, with a stable flame being defined
as either “attached stable” or “detached stable”.
Note
See G.2.2 for further guidance on flame stability.
G.2.2.1 Introduction
Flame stability of the flare is critical to safe and reliable operation and for assuring proper destruction of volatile and
combustible components in the flare gas. Flare gas composition and exit velocity are important considerations in
flare burner design and stable operation.
There are four stages of observable flame stability:

Stage 1—attached stable,

Stage 2—detached but stable flame,

Stage 3—detached unstable flame,
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
Stage 4—flame-out.
G.2.2.2 Design Considerations
Judgment of flame stability is best undertaken at night when blue sections of a flame are more visible.
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Both Stages 1 and 2 are acceptable flames. Stage 3 is a flame in transition from Stage 2 to Stage 4 or from Stage 4
to Stage 2. Stage 3 flaring is often accompanied by a pounding low frequency noise. Burning Stage 2 can be
accomplished using flame retaining devices located at or near the flare gas exit. Stability can also be achieved by
using turbulence created by gas jets located at or near the exit to stabilize the flame.
G.3 Combustion Methods
G.3.1 Flares with Smoke Suppression (5.7.3.2)
Normative Requirements for Consideration:
 Smokeless operational range shall be as defined in the flare burner design sheet.
 Smokeless operation shall be as defined by opacity or Ringelmann number
 Other operating requirements for the flare eg noise and regulatory requirements shall be set in the flare
datasheet
G.3.1.1
Introduction
Smokeless operation is normally the overriding requirement when designing the burner for a flare system. Almost
every flare design is aimed at inducing smokeless operation under a certain set of flare gas or utility availability
conditions.
To promote even air distribution throughout the flames (and thus prevent smoke formation), energy is required to
create turbulence and mixing of the combustion air within the flare gas as it is being ignited. This energy can be
present in the gases, in the form of pressure and velocity, or it can be exerted on the system through another
medium, such as injecting high-pressure steam, compressed air or low-pressure blower air into the gases as they
exit the flare burner.
G.3.1.2
Design Considerations

Assist medium (e.g. steam or air) consumption at turndown firing rates below the smokeless design point may
need to have a higher assist medium-to-hydrocarbon ratio than the smokeless design condition to mix
sufficient air with the flare gas.

To create conditions favorable for smokeless combustion, flare designs range in complexity from a simple
open pipe with an ignition source to integrated, staged flare systems with complex control systems.

Smoke-free operation of flares can be achieved by various methods, including steam injection, injection of
high-pressure waste gas, forced draft air, operation of flares as a premix burner, or distribution of the flow
through many small burners.

A common type of smokeless flare involves steam injection.
G.3.2 Steam Requirements for Steam Assisted Flares (5.7.3.2.2)
Normative Requirements for Consideration:
 Composition of the gas to be burned shall be listed on the flare datasheet for all defined operational smokeless
cases.
 The manufacturer shall define the minimum necessary steam rate required for smokeless flare operation at
required design conditions. This shall be documented on the flare datasheet.
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G.3.2.1
Introduction
FO
The amount of steam required for smokeless burning depends on the vapor flow rate to be burned and the detailed
composition of the mixture. Key parameters involving smokeless combustion include percentage of unsaturates,
percentage of inerts, and the mixture relative molecular mass. Certain specific compounds require special
consideration by the vendor. Examples include ethylene, butadiene, acetylene, and ethylene oxide. Datasheets from
Annex F provide a convenient means for specifying the composition.
G.3.2.2
Design Considerations
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Table G.1 may be used to estimate steam requirements as a function of composition. For mixtures, the estimate
of steam requirements can be proportioned based on the specific mass fraction of hydrocarbon component in the
mixture (i.e. exclude hydrogen sulfide, inerts).
In any event, if a proprietary smokeless flare is purchased, the manufacturer should be consulted about the
minimum necessary steam rate.
Analysis techniques to define the steam conditions that result in high combustion efficiency is actively being
researched at this time, but a universally accepted method has yet to be identified.
Table G.1-Suggested Steam Injection Rates
Gases Being Flared
ab
Approximate Steam Rate
kg (lb) of steam per kg (lb) of hydrocarbon gas
Paraffins
Ethane
0.10 to 0.15
Propane
0.25 to 0.30
Butane
0.30 to 0.35
Pentane plus
0.40 to 0.45
Olefins
Ethylene
0.40 to 0.50
Propylene
0.50 to 0.60
Butene
0.60 to 0.70
Diolefins
Propadiene
0.70 to 0.80
Butadiene
0.90 to 1.00
Pentadiene
1.10 to 1.20
Acetylenes
Acetylene
0.50 to 0.60
Aromatics
a These
Benzene
0.80 to 0.90
Toluene
0.85 to 0.95
Xylene
0.90 to 1.00
suggested steam factors have units of mass steam/mass hydrocarbon. Because
flare gas can have significant quantities of hydrogen and nitrogen, it is important to
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account for this when using these factors by adjusting for the hydrocarbon content of the
flare gas:
 wi   API i
i=1
n
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Suggested Steam-to-flare-Gas Ratio =
where
n
is the number of components in flare gas mixture. Both hydrocarbon and
nonhydrocarbons are included in this count;
w(i) is the weight fraction of component (i) in the flare gas mixture;
API(i)
is the steam-to-hydrocarbon mass ratio for hydrocarbon (i) from this
table. Note that nonhydrocarbons such as hydrogen, hydrogen sulfide, carbon
monoxide, ammonia, nitrogen, carbon dioxide, etc. would have a steam ratio
value of zero.
b These values provide only a general guideline because they can be affected by flare tip
design configuration, gas flow rate, gas composition, gas pressure, steam injector design,
steam pressure, steam velocity, steam control, sequence order of steam injection, gas
velocity, inerts, wind speed effect (depending upon the flow regime flame that is present),
flare equipment condition, etc.
G.3.3 Degree of Smokelessness (5.7.3.2.3)
Normative Requirements for Consideration:
 The level of smokeless operation required for different operating conditions shall be specified on the flare data
sheet and consider regulatory requirements.
G.3.3.1
Introduction
The flare may be designed for various degrees of smokelessness.
G.3.3.2
Design Considerations
Many state and federal regulations state the smokeless requirement in the form “No operator shall allow the flare
emissions to exceed 20 % opacity for more than 5 min in any consecutive 2 h period.” This type of regulation is
usually the basis for designing flares to achieve Ringelmann 1 (20 % opacity) performance.
Other applications can require Ringelmann 0 (zero opacity) for regulatory or community relations reasons. It is
necessary for the user to understand the local regulatory requirements that govern smokeless requirements.
G.3.4 Steam Injection (5.7.3.2.4)
Normative Requirements for Consideration:
 Maximum steam limits shall be specified and consider turndown conditions.
G.3.4.1
Introduction
Flare burners that use steam to control smoking are a common for elevated single point flares. The steam can be
injected through a single pipe nozzle located in the center of the flare, through a series of steam/air injectors in the
flare, through a manifold located around the periphery of the flare tip or a combination of all three, as appropriate
for the application [see Figure G.1, a) and b)]. The steam is injected into the flame zone to create turbulence
and/or aspirate air into the flame zone via the steam jets.
The improved air distribution allows the air to react more readily with the flare gases to eliminate the fuel-rich
conditions that result in smoke formation. Another factor assisting smokeless operation is the steam water-gas
shift interaction where carbon monoxide and water vapor react to form carbon dioxide and hydrogen, which is
more easily burned. Proprietary flare burner designs that offer unique steam injection methods and varying
required steam-to-hydrocarbon ratios are available from various flare vendors.
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the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
G.3.4.2
Design Considerations
FO
The suggested amount of steam that should be injected into the gases being flared to promote smokeless burning
(Ringelmann Number of zero) can be estimated from Table G.1 for several components. These values provide
only a general guideline because they can be affected by flare burner design, relief or waste gas flow rate,
composition, pressure, steam velocity, gas velocity, etc. The flare vendor should be consulted for a detailed
assessment of steam requirements for smokeless combustion at varying flare operating loads and process
conditions.
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Although increasing the steam to flare gas ratio would benefit smoke control, it should be noted that a flare can be
oversteamed. If too much steam is added, the combustion efficiency of the flare will decrease, particularly during
turndown conditions.
While there is usually a significant range of steam-to-flare-gas ratios that will produce high combustion efficiencies,
the combustion efficiency declines abruptly when excessive steam to flare gas ratios are applied. The upperboundary of steam-to-flare-gas ratio to maintain a high combustion efficiency is affected by the many factors
noted above.
Currently, there is insufficient information to provide specific guidance on upper steam limits in this standard. It
should be noted that Table G.1 is not intended to specify maximum steam rates for any defined combustion
efficiency. The steam rates and combustion efficiencies vary as a function of the process design conditions and
the flare burner proprietary design.
Although steam is normally provided from a supply header at 700 kPag to 1000 kPag (approximately 100 psig to
150 psig), special designs are available for utilizing steam pressure as low as 200 kPag (~30 psig). The major
impact of lower steam pressure is the need for a higher steam-to-hydrocarbon ratio during smokeless turndown
conditions. The steam pressure at the point of injection directly affects air inspiration. Hence, steam piping
pressure drop from the supply header to the injection point should be considered in designing the piping layout.
G.3.5 High-pressure Air (5.7.3.2.5)
G.3.5.1
Introduction
High-pressure air can also be used to prevent smoke formation as a substitute for steam, however is less effective.
Air used like steam can educt air into the combustion zone and promote mixing at a reduced efficiency. This
approach is less common because compressed air is usually more expensive than steam. However, in some
situations with low smokeless capacities, it can be preferable, for example, in arctic or low-temperature applications
where steam can freeze and plug the flare tip/stack. Also, other applications include desert or island installations
where there is a shortage of water for steam, or where the waste-flare gas stream reacts with water
G.3.4.2
Design Considerations
The same injection methods described for steam (see G.3.4) are used with compressed air. The air is usually
provided at a pressure of 689 kPag (100 psig). The mass of air required is approximately 1.2 times the steam
mass, because the compressed air does not produce the water-gas shift reaction that occurs with steam.
In general, high-pressure air assist is useful for retrofit of an existing flare or where the required smokeless flow is
small, and steam is not economically available
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< upper steam ring
Figure G.1- Steam-injected Smokeless Flare Burners
G.3.6 High-pressure Water (5.7.3.2.6)
Normative Requirements for Consideration:
 Where required, freeze protection shall be installed for water systems as defined in the flare data sheet.
G.3.6.1
Introduction
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High-pressure water, while quite uncommon, is also used to control smoking, especially for horizontal flare
applications and when it is necessary to eliminate large quantities of waste water or brine. A water to flare gas ratio
of approximately 1:1 is typically used, e.g. 0.45 kg (1 lb) of water at 350 kPag to 700 kPag (approximately 50 psig to
100 psig) is usually required for each 0.45 kg (1 lb) of gas flared.
Design Considerations
FO
G.3.6.2
Freeze protection of high-pressure water lines in cold climates is required.
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Due to the difficulty in controlling the water flow at low flaring rates, a staged water-spray injection system is
usually specified.
G.3.7 Low-pressure Forced-air System (5.7.3.2.7)
G.3.7.1
Introduction
A low-pressure forced-air system is usually the first alternative evaluated if insufficient on-site utilities are available to
aid in producing a smokeless operation. The system creates turbulence in the flame zone by injecting low-pressure
air supplied from a blower across the flare tip as the gases are being ignited, thus promoting even air distribution
throughout the flames. Typically, air at a gauge pressure of 0.5 kPa to 5.0 kPa (2 in. H2O to 20 in. H2O) flows
coaxially with the flare gas to the flare tip where the two are mixed.
G.3.7.2
Design Considerations
A low-pressure forced-air system has a higher initial cost due to the requirement for a dual stack and an air blower.
See Figure G.2. However, this system has a much lower operating cost relative to a steam-assisted design
(requiring only power for a blower).
The additional quantity of air supplied by the blower for smokeless operation is normally 10 % to 30 % of the
stoichiometric air required for saturated hydrocarbons and 30 % to 40 % of the stoichiometric air required for
unsaturated hydrocarbons.
169
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Figure G.2-Typical Forced-air Assisted Smokeless Flare
G.3.8 High-pressure Flaring (5.7.3.2.8)
Normative Requirements for Consideration:
 Staged tips shall be appropriately spaced to allow air entry into the system.
G.3.8.1
Introduction
170
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A high-pressure system does not require any utilities such as steam or air to promote smokeless flaring. Instead,
these systems utilize pressure energy available within the flare gas itself [typically 35 kPag to 140 kPag (5 psig to
20 psig) minimum at the flare tip] to eliminate fuel-rich conditions and resulting smoke within the flames.
G.3.8.2
Design Considerations
FO
High-pressure system limitations are also present but vary by manufacturer and nature of design. By injecting the
flare gas into the atmosphere at a high pressure, turbulence is created in the flame zone, which promotes even air
distribution throughout the flames.
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Since no external utilities are required, these systems are normally advantageous for disposing of very large gas
releases, both from the economics of smokeless operation and the control of flame shape.
Single-point flare burners used in high-pressure applications are relatively small in capacity, with larger system
designs requiring a multi-burner flare designed in a manifolded arrangement and often a pressure staged design.
Maintaining sufficient pressure at the burner during turndown conditions is critical and often requires that a staging
system be employed to proportionately control the number of flare burners in service relative to the amount of gas
flowing.
Multi-burner staged-flare systems can be mounted either at grade or elevated with the larger systems requiring a
ground-level design given the large number of individual flare burners that are often required. It is not uncommon
to have more than 300 burners in a large ground level staged-flare system. Evenly spaced burners are required to
allow adequate air entry into the system for proper smokeless combustion.
Staged flares should provide backup for system failures by inclusion of bypasses or emergency vents. A failsafe
pressure-relief bypass around individual staging control valves are a commonly used safety measure. A rupture
disk or similar device is typically used as a failsafe pressure-relief bypass.
If environmental regulations allow, high-pressure flares have been used in burn-pit applications to burn liquids.
G.3.9 High-pressure Flaring (5.7.3.2.9)
G.3.9.1
Introduction
High-pressure fuel gas can also be used to prevent smoke formation by entraining outside air into the flare flame and
generating turbulence to assist overall combustion.
G.3.9.2
Design Considerations
Typically, high-pressure fuel gas injection methods are similar to steam assisted flare burners, however special
high-performance flare burners are used to reduce the amount of assist gas.
If natural gas is used as the assist gas, typically 0.23 kg to 0.34 kg (0.5 lb to 0.75 lb) of assist gas per kg (lb) of
flare gas is required, based on a flare gas consisting of normal paraffins such as propane and butane. The supply
pressure for natural gas assist is typically 500 kPag (~75 psig) (minimum) with 1000 kPag (~150 psig) preferred.
G.3.10 Control of Fluid Injection for Smoke Suppression (5.7.3.2.10)
Normative Requirements for Consideration:
 At low flaring rates, fluctuations in either pressure or flow are so minute that very sensitive instruments are
required to provide sufficient steam for smokeless combustion while at the same time avoiding waste, controls
shall be selected to minimize excessive steam use.
G.3.10.1
Introduction
The following methods of controlling steam (or compressed air, high-pressure water or fuel gas) for smokeless flare
control are commonly applied with many other control strategies possible.
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
Manual Operation: Manual control usually involves remote operation of a steam valve by operating personnel
assigned to a unit from which the flare is readily visible. This method is satisfactory if short-term smoking can
be tolerated when a sudden increase in flaring occurs. With a manual arrangement, close supervision is
required to ensure that the steam flow is reduced following the correction of an upset. Operating costs can be
excessive if monitoring is not timely.
FO

Video Monitoring with Manual Control: The philosophy is the same with manual operation except that a video
monitoring system is added so that the control room operators can monitor and control the steam flow more
effectively.
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Feed Forward Control System for Pressure, Mass Flow, or Velocity: By measuring the amount of flare gas
flowing to the flare, the steam rate can be automatically adjusted to compensate for rate changes. This
system might not be desirable if the composition of the gas being flared varies widely over time e.g., paraffins
to olefins or aromatics, hydrogen, or various mixtures thereof.

Feedback System Using an Infrared Sensor: Infrared sensors can be used to detect smoke formation in the
flames and automatically adjust the steam control valve to compensate. A disadvantage of this system is that
infrared waves are absorbed by moisture and the resultant feedback signal is reduced in rainy or foggy
conditions.
G.3.10.2
Design Considerations
Proper steam (or other assist media) control is important to achieve the maximum reaction-efficiency potential oh
the flare burner.
Proper steam control is particularly true at low flaring rates where it is possible to overaerate the flare flame to the
point of near flame extinguishment. Such operation may be based on the belief that it is preferable to overaerate a
flame to a point that it becomes difficult to see. This premise is not correct. Studies and tests of flare reaction
efficiency have established that an over-steamed flare has a lower reaction efficiency than a properly aerated flare
flame.
G.3.11 Noise Caused by Smokeless Flaring (5.7.3.2.11)
G.3.11.1
Introduction
Smokeless flares using steam can produce noise due to excessive steam usage
G.3.11.2
[147)
.
Design Considerations
As the amount of steam is increased, flame instability and pulsation (flame on/flame off conditions) can occur.
The noise associated with the oversteaming tends to be low frequency and carry for long distances. Even further
oversteaming could result in a flame-out.
Smokeless operation using compressed air can also produce pulsation but is less likely since the air does not
have the same cooling (quenching the flame) as the steam.
Flares using high-pressure fuel gas assist generally do not have this pulsation or flame-out problem due to the
secondary flame produced by the assist gas ring or burners.
Air blower flares can produce a similar flame quenching with resulting flame instability and low frequency pulsation
when large amounts of excess air are used. Air blower flares are less likely to produce this problem due to the
good gas and air distribution.
Improper sequencing of the steam on a multiple steam injection tip/burner can cause noise associated with pulsation.
If the upper steam ring is used before the lower steam-air tubes, the upper steam may “cap” the flare tip outlet
resulting in flare instability and pulsation. This “cap” steam rate itself may not be at excessive steam rates compared
to the flare gas flow rate but flame instability and pulsation may still occur.
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the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
G.3.12 Noise Caused by Smokeless Flaring (5.7.3.3)
Normative Requirements for Consideration:
FO
 Flare burners of this style shall include a flame-retention device (to increase flame stability at high flow rates)
and one or more pilots (depending upon the diameter of the burner).
G.3.12.1
Introduction
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The simplest flare burner design is commonly referred to as a utility or pipe flare burner and can consist of little more
than a piece of pipe fitted with a flame retention device for flame stability at higher exit velocities (the upper portion is
typically stainless steel to endure the high flame temperatures) and a pilot system for gas ignition.
G.3.12.2
Design Considerations
A utility flare burner is a plain design that has no special features to prevent smoke formation, and consequently
should not be used in applications where smokeless operation is required, unless the gases being flared, such as
methane or hydrogen, are not prone to smoking.
Flare burners of this style should include a flame-retention device (to increase flame stability at high flow rates)
and one or more pilots (depending upon the diameter of the burner).
Windshields or heat shields are usually added on flare burners to reduce flame lick on the outside body of the
burner.
An inner refractory lining is also common with larger diameter tips to minimize thermal degradation caused by
internal burning at low rates (known as burnback).
G.3.13 Flaring of Gases with Low Heating Value (5.7.3.4)
Normative Requirements for Consideration:
 Gases with low heating value shall have low exit velocities to avoid creation of high excess air conditions that
could over aerate a combustible mixture causing incomplete combustion and flame instability (see G.2.2)
G.3.13.1
Introduction
All the preceding descriptions for combustion methods have been for flare burners used in the disposal of
3
exothermic flare gases; that is, gases that have sufficient heating value (usually greater than 7.5 MJ/m (200 Btu/scf)
3
for unassisted flares and 11.2 MJ/m (300 Btu/scf) for assisted flares) for self-sustaining combustion without any fuel
gas addition.
Flares gases with heating values below the threshold for self-sustaining combustion require fuel gas additions. Fuel
gas assisted flares are generally referred to as endothermic flares.
G.3.13.2
Design Considerations
Endothermic gases can be disposed of in thermal incineration systems; however, there are situations where the
preferred approach is to use a special flare design. These flares utilize auxiliary fuel gas to burn the flare gases.
With small gas flow rates, simple enrichment of the flare gases by adding fuel gas in the flare header to raise the
net heating value of the mixture can be sufficient. In other situations, it can be necessary to add a fuel gas
injection manifold around the flare tip (similar to an upper steam ring) and build a fire around the exit end of the
flare tip through which it is necessary for the gases to flow.
Dilute ammonia or high CO2 composition flare gases with small amounts of H2S are common applications where
the addition of fuel gas is required.
Gases with low heating value shall have low exit velocities to avoid creation of high excess air conditions that
could overaerate a combustible mixture causing incomplete combustion and flame instability (see G.2.2).
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G.4 Flare System Design
G.4.1 Ground Flares (5.7.4.1)
G.4.1.1 Introduction
FO
Ground flares encompass a broad range of vastly different types of flare systems. In general, any of the flare tips or
systems discussed in G.3.1 through G.3.13 can be mounted atop an elevated stack or mounted at grade.
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G.4.1.2 Design Considerations
With increasingly strict requirements regarding flame visibility, emissions, and noise, enclosed ground flares can
offer the advantages of hiding flames, monitoring emissions, and lowering noise.
However, the initial cost often makes them undesirable for large releases when compared to elevated systems.
With an enclosed ground flare system, a variety of burners may be utilized and are enclosed or hidden behind a
refractory-lined carbon steel shell.
A significant disadvantage with a ground flare is the potential accumulation of a vapor cloud in the event of a flare
malfunction. As a result, special safety dispersion systems are usually included in the ground flare system. For
this reason, instrumentation for monitoring and controlling ground flares is typically more stringent than for an
elevated system.
These flares are typically the most expensive because of the size of the shell or fence and the additional
instrumentation that can be required to monitor these key parameters.
Another significant limitation is that enclosed ground flares have significantly less capacity than elevated flares.
If emissions monitoring is not required, a fenced ground flare system can be designed with very large capacity. A
radiation/wind fence can partially or totally hide the flames from view to a person located near grade. By restricting
the amount of flame visible to a point of interest at grade level locations, it is possible to greatly reduce the
external radiation load from the flare. Fenced ground flares frequently use multiple, high-pressure burners to
obtain smokeless performance at firing rates that cannot normally be handled smokelessly by elevated flares.
[8]
A complete description of an enclosed ground flare can be found in API 537 , where an entire chapter is devoted
to this type of flare.>>>RW: Not true Options exist for bottom or side firing, staged or unstaged control, and
steam-assisted, air-assisted, pressure-assisted, or nonassisted burners.
This type of flare system is often relatively complex and may involve many independent burner systems.
Accounting for the various interactions between burners, fences, stack, smoke suppression equipment, piping,
and wind/weather requires considerable experience and can involve detailed flow modelling before a workable
design can be achieved.
G.4.2 Elevated Flares (5.7.4.2)
G.4.2.1 Introduction
The most common type of flare system currently in use is an elevated flare. In these systems the flare burner is
mounted atop the stack, which reduces ground-level radiation and improves the toxicity-dispersion profile. There
are three common stack support methods: self-supported, guy-wire supported, and derrick supported.

Self-supported stacks are often used for shorter stacks with limited plot space. They are normally limited to a
stack height 100 m (~330 ft). See Figure G.3 a).

Guy-wire-supported stacks typically require larger land area than self-supported or derrick-supported stacks.
Extreme process temperatures require special considerations when designing guy-wires. The typical guy-wire
radius is equal to one-half the overall stack height. Guyed stack height is normally limited to a stack height of
250 m (~800 ft). See Figure G.3 b).
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
Derrick-supported stacks are used only on larger stacks where self-supported design is not practical or
available land area excludes a guy-wire design. Some derrick designs allow the flare stack and tip to be
lowered to grade on movable trolleys for inspection and maintenance. This self-lowering design is especially
useful when multiple stacks are installed on the same derrick. In locations where land is not available, the
multiflare derrick can be used. See Figure G.3 c).
FO
Additional information regarding elevated flares and support structures and their structural design can be found in
API 537
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Figure G.3-Flare Structures
G.4.2.2 Design Considerations
None
G.4.3 Unassisted Pipe Flares (5.7.4.1)
Normative Requirements for Consideration:
 The relief gas discharging from the flare tip shall occur within the hydraulic design for the flare system (within the
allowable pressure drop and flame combustion velocity limits).
 It shall be ignited and burned with the designed flame characteristics
G.4.3.1 Introduction
An unassisted pipe flare is used where smokeless burning assist is not required. Pilots and a pilot ignition system
provide flare flame ignition.
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The pipe flare burner may have a mechanical device (e.g. flame retention ring) or other means of establishing and
maintaining a stable flame. The ignition fire from the gas discharge is initially ignited by interaction with the pilot(s)
flames. Once the pilot lights and the flame stabilize, the flare should maintain flame stability over the operating
design range. Flame stability for a pipe flare is primarily dependent upon the selection of the gas exit velocity.
FO
G.4.3.2 Design Considerations
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The form of the flame produced by an unassisted pipe flare is a function of the relief gas composition and the gas
exit velocity. At greater gas exit velocities, the flame uses the gas discharge energy to pull combustion air into the
flame. It produces a shorter, more erect flame that has greater resistance to wind deflections. At lower gas exit
velocities, air is drawn to the flame by the buoyancy of the heated products of combustion. A buoyant flame is
typically softer, longer and more affected by wind than a flame that uses higher gas exit velocities.
Low gas exit velocities and buoyant flames are preferred for successful combustion of low heating value relief gas.
High gas exit velocities are preferred for higher heating value hydrocarbon relief gases or for relief gases rich in
hydrogen. Because of the high flame velocity, wide flammability range, buoyancy effects and noise, hydrogen
flares require special design considerations. The manufacturer should be consulted for details.
Flare combustion noise is influenced by gas exit velocity. Increased relief gas exit velocity can produce greater
combustion turbulence and higher combustion noise. The highest combustion-noise levels are realized when a
flare tip operates at gas exit velocities where combustion instabilities occur.
The relief gas discharging from the flare tip shall occur within the hydraulic design for the flare system (within the
allowable pressure drop and flame combustion velocity limits). It shall be ignited and burned with the designed
flame characteristics.
G.4.4 Burn Pits (5.7.4.4)
G.4.4.1 Introduction
Burn-pits normally require excavation or bermed areas to contain liquid hydrocarbons or other objectionable
materials produced by incomplete combustion.
G.4.4.2 Design Considerations
Seepage from a poorly designed or maintained burn-pit can pose a threat to groundwater supplies.
G.5 Sizing
G.5.1 General (5.7.5.1)
Normative Requirements for Consideration:
 Flame retainers that restrict flow area by 2 % to 10 %, which should be accounted for in the flare and header
sizing.
G.5.1.1 Introduction
Factors governing the sizing of flares are addressed in G.5.2 and G.5.3. General considerations involved in the
calculation of these requirements are discussed in 5.7.1 through 5.7.4 of API Standard 521, Sixth Edition.
G.5.1.2 Design Considerations
Examples covering the full design of a flare stack are given in C.2, API Standard 521 Sixth Edition.
Note
Flare diameter calculations are based on a basic flare.
Most commercial flare burners include flame retention devices that restrict the relief gas flow area by 2 % to 10 %.
Flow area restrictions should be accounted for in the flare and header sizing.
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publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
G.5.2 Flare Riser Diameter (5.7.5.2)
G.5.2.1 Introduction
The flare riser diameter is generally sized on a velocity basis, although pressure drop should be checked.
FO
G.5.2.2 Design Considerations
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A riser velocity (not flare burner velocity) of up to 0.5 Mach is typically allowed for a peak, short-term, infrequent
flow for both low- and high-pressure flares. This depends on the following:

volume ratio of maximum conceivable flare flow to anticipated average flare flow;

probable timing, frequency, and duration of those flows;

design criteria established for the project to stabilize flare burning.
Sonic velocity operation can be appropriate for high-pressure flare tips but not within the flare riser.
Smokeless flares should be sized for the conditions under which they are to operate smokelessly. Equation 32 or
Equation 33 in API Standard 521, Sixth Edition, can be used to calculate the Mach number See 5.5.5,
API Standard 521, Sixth Edition.
Velocity limitations imposed by regulatory agencies (see Bibliographic Item [59]) may not apply to flares in
emergency relief service.
Pressure drops as large as 14 kPa (2 psi) have been satisfactorily used at the flare burner. Modern conventional
flare burners with proper flame stabilization can operate well above this level.
Most flare vendors also have a line of special-duty high-pressure flares that can operate around 700 kPag
[8]
(100 psig) or higher. This general class of flare designs is recognized by API 537 and usually operates
smokelessly without steam or air assistance.
Too low a flare burner exit velocity can cause heat and corrosion damage. The burning of the gases becomes
quite slow, and the flame is greatly influenced by the wind. The low-pressure area on the downwind side of the
stack can cause the burning gases to be drawn down along the stack for 3 m (10 ft) or more.
Mechanical details of flare burners relating to stack downwash effect can be found in API 537.
G.5.3 Flare Stack Height (G.5.3)
Normative Requirements for Consideration:
 A dispersion analysis may be used to aid calculating the required flare height in the event of a flare
flame being extinguished.
 For flare radiation a number of techniques are available to set the height of the flare against
recommended radiation levels required. At least one of the techniques listed shall be used.
G.5.3.1 Introduction
The flare stack height is generally based on the radiant-heat intensity generated by the flare flame.
G.5.3.2 Design Considerations
From the vendor obtain radiation isopleths for their best technology flare burner for a given flow rate, composition,
temperature and pressure.
For radiant heat intensity Equation (45) in 5.7.2.3.3, API 521, Sixth Edition applies. The recommended levels of
radiation intensity, K, are given in Table 12, API 521, Sixth Edition.
177
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
The quality of combustion affects the radiation characteristics. Use of the fraction of heat radiated, F, based on
the U.S. Bureau of Mines data given in Table 13, API 521, Sixth Edition, is considered to result in a reasonable
but conservative stack height.
FO
Another factor to be considered is the effect of wind in tilting the flame, thus varying the distance from the center
of the flame, which is considered to be the origin of the total radiant-heat release, with respect to the plant location
under consideration. A generalized curve for approximating the effect of wind is given in Figure 7, API 521, Sixth
Edition
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Where there is concern about the resulting atmospheric dispersion (both flammable and toxic) if the flare were to
be extinguished, dispersion analyses (see 5.7.10.2, API 521, Sixth Edition) may be used to calculate the probable
concentration at the point in question and required flare height.
G.5.4 Flare Burner Pressure Drop (5.7.5.4)
Normative Requirements for Consideration:
 The pressure-drop across the flare tip including accessories (e.g. seals) shall be included in the hydraulic
assessments to ensure backpressure limitations are not exceeded
G.5.2.1 Introduction
The pressure-drop across the flare tip including accessories (e.g. seals) is needed in the hydraulic assessments to
ensure backpressure limitations are not exceeded.
G.5.2.2 Design Considerations
The pressure drop across the flare burner will vary depending upon burner type, size selection, mass flow and
fluid flow conditions properties.
Review of all operating cases should be done to ensure definition the design case related to system back
pressure. The use of Veq is a quick method to compare all operating case to determine which ones may be
controlling developed back pressure. See Annex E.2.
Higher pressure drops across the flare burner generally promote combustion performance (including smokeless
performance) but increase the backpressure on the pressure -relief device. The flare burner the manufacturer
should be consulted flare burner hydraulic assessments.
G.6 Purging
G.6.1 General (5.7.6.1)
Normative Requirements for Consideration:
 Buoyance seals: The drain shall be kept open and protected from freezing in cold climates.
 Velocity seals: A hole shall be made in each baffle plate to permit drainage of possible liquids in order to avoid
corrosion and/or freezing.
G.6.1.1 Introduction
For safety purposes, a precommissioning purge and subsequent continuous purge with a noncondensable oxygenfree gas is required through the flare system.
G.6.1.2 Design Considerations
G.6.1.2.1
General:
For safety purposes, a precommissioning purge and subsequent continuous purge with a noncondensable
oxygen-free gas is required through the flare system.
178
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
The prepurge displaces any existing air from the stack and the continuous purge ensures that atmospheric air
does not enter the stack through the flare tip during low-flow conditions. There should, then, be a continuous
purge of auxiliary gas, which may be gas from normal process vents (provided that the required flow rate can be
maintained).
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The requirements for a continuous purge can be eliminated if a liquid seal is located near the base of the stack
(see 5.7.8, API 521, Sixth Edition). This requires special precautions in the design of the stack to assure viability
in the event of an internal explosion. It also can allow air to infiltrate to the liquid seal, which, for some seal
mediums, carries other requirements.
Air present in the stack can create a potentially explosive mixture with incoming flare gas during low-flare gas flow
rate conditions.
There are two common types of purge-reduction devices, usually located at/or below the flare tip, that are used to
reduce the amount of continuous purge gas required to prevent air infiltration into the flare stack: the buoyancy
seal and the velocity seal.
G.6.1.2.2
Buoyance Seal:
The buoyancy seal uses the difference in the relative molecular masses of the purge gas and air to form a gravity
seal that, at the proper purge gas flow, prevents the air from entering into the stack.
A baffled cylinder arrangement forces the incoming air through two 180° bends (one bend up and one bend down)
before it can enter into the flare stack.
If the purge gas is lighter than air, the purge gas accumulates in the top of the seal and prevents the air from
infiltrating the system.
If the purge gas is heavier than air, the purge gas accumulates in the bottom of the seal and prevents air from
infiltrating.
This seal normally reduces the purge gas velocity required through the tip to 0.003 m/s (0.01 ft/s). Also, with most
purge gas compositions, this rate limits oxygen levels below the device to less than 0.1 %.
Higher purge gas velocities can be required to avoid burnback within the flare burner.
The two 180° turns in a buoyancy seal can cause liquid collection in the seal (see Figure 11) in which case a drain
equipped with a loop seal (for example) is required. There is a potential for plugging of the buoyancy seal due to
corrosion products, combustion products (coke), water freezing, or refractory debris from the refractory-lined flare
tip resulting in an unsafe condition. The drain shall be kept open and protected from freezing in cold climates.
G.6.1.2.3
Velocity Seal
The velocity seal works under the premise that infiltrating air enters through the flare burner and hugs the inner
wall of the flare burner.
The velocity seal is a cone-shaped obstruction, with single or multiple baffles, which forces the air away from the
wall. It subsequently encounters the focused purge gas flow and is swept out of the burner.
This seal normally reduces the purge gas velocity through the tip to between 0.006 m/s to 0.012 m/s (0.02 ft/s and
0.04 ft/s), which keeps oxygen concentrations below the seal to 4 % to 8 % (~50 % of the limiting oxygen
concentration required to create a flammable mixture).
Higher purge gas velocities can be required to avoid burnback within the flare tip. Caution should be exercised
when the waste-gas stream can contain hydrogen, ethylene, or other gases with wide explosive limits. In such
cases, a higher purge rate can be required to avoid an explosive mixture with air.
A hole should be made in each baffle plate to permit drainage of possible liquids in order to avoid corrosion and/or
freezing.
179
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
Without either one of these seals, the purge gas velocity in the tip required to prevent air infiltration into the stack
should be determined using the procedure described in 5.7.6.2, API 521, Sixth edition.
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Figure G.4-Purge reduction Device – Buoyancy Seal
G.6.2 Air Infiltration/Continuous Purging Requirements for Stacks Without a Purge
Reduction Seal (5.7.6.2)
Normative Requirements for Consideration:
 If purge gas is required, the user shall assure the reliability of its supply
G.6.2.1 Introduction
Air infiltration down the flare stack from wind or density effects can be mitigated by use of purge gas.
G.6.1.2 Design Considerations
If purge gas is required, the user shall assure the reliability of its supply.
The amount of purge gas required can be reduced by the use of a purge-reduction seal (e.g. buoyancy seal or
velocity seal; see G.6.1).
180
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
For lighter-than-air purge gases, Equation G.1 and Equation G.2 can be used to determine Q, the purge gas rate,
3
3
expressed in normal m /h (standard ft /h) for continuous purge requirements in open flares without the effect of
[86] [87]
buoyancy seal or velocity seal
.
In SI units:
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(G.1)
In USC units:
(G.2)
where
D
is the flare stack diameter, expressed in m (in.);
yis the column depth at which the oxygen concentration (O 2) is predicted, expressed in meters (feet);
O2
is the oxygen volume fraction, expressed as a percentage;
Ci
is the volume fraction of component i, a number between 0 and 1;
Ki
is a constant for component i.
The following are typical values for Ki (independent of wind except where noted).
Hydrogen:
K = +5.783.
Helium: K = +5.078.
Methane: K = +2.328.
Nitrogen: K = +1.067 (no wind).
Nitrogen: K = +1.707 [with a wind speed of approximately 7 m/s (15 mph)].
Ethane: K = –1.067.
Propane: K = –2.651.
CO2: K = –2.651.
C4+: K = –6.586.
NOTE 1
Steam or other condensables are not suitable purge gases.
NOTE 2
The reference temperature for standard conditions [15.6 °C (60 °F)] is not the same as the reference temperature
for normal conditions [0 °C (32 °F)]. The conversion between standard and normal conditions has been incorporated when
reporting the results in the different unit systems. The user is cautioned that the volumetric rates reported in the different unit
systems might not appear to be equivalent because of this temperature conversion.
Equation G.1 and Equation G.2 can be simplified to Equation G.3 and Equation G.4 using the typical criteria of
limiting the oxygen volume fraction to 6 % at a distance of 7.62 m (25 ft) down the flare stack (except that lower
oxygen concentrations should be used for certain compounds such as hydrogen):
In SI units:
181
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publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
Q = 31.25D3.46  K
(G.3)
In USC units:
FO
Q = 0.0035283D3.46  K
(G.4)
where
3
3
is the purge gas rate, expressed in normal m /h (standard ft /h);
D
is the flare stack diameter, expressed in meters (inches);
K
is a constant (see above).
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[40]
Based on recent test data involving natural gas production facility flares , a significant reduction in purge rates
as predicted by Equations G.1, G.2, G.3, and G.4 may be taken under certain constraints.
The user is cautioned not to extrapolate outside the bounds and conditions under which the tests were conducted.
If the purge gas is heavier than air, then no significant positive buoyancy force exists. For this condition, the purge
[87]
rate for nitrogen should be used .
If the gas in the stack (e.g. hydrogen or methane) is lighter than air, the pressure in the bottom of the stack can be
lower than atmospheric, even with some outflow from the top of the stack. This condition creates a situation in which
any flange leaks, open drains/vents or other openings in the flare header draws air into the flare header, resulting in
potential for an internal explosion. For this reason, the users are cautioned to maintain the integrity of their
equipment and follow proper safety precautions when opening an active header.
G.7 Ignition of Flare Gases
G.7.1 General (5.7.7.1)
G.7.1.1 Introduction
To ensure ignition of flare gases, continuous pilots with means for remote ignition are recommended for all flares.
G.7.1.2 Design Considerations
Some regulations can require the presence of a continuous pilot flame to be proven by thermocouple or
equivalent means.
The most commonly used type of igniter is the flame-front propagation type, which uses a spark from a remote
location to ignite a flammable mixture.
Pilot-igniter controls are located near the base of elevated flares and at least 30 m (100 ft) from ground flares (see
[8]
G.7.3). An extensive discussion of pilots and ignition systems can be found in API 537 .
G.7.2 Pilot Fuel Gas Supply (5.7.7.2)
G.7.2.1 Introduction
The fuel gas supply to the pilots and igniters should be highly reliable.
G.7.2.2 Design Considerations
Since normal plant fuel sources can be upset or lost, it is desirable to provide a backup system connected to the
most reliable alternative fuel source, with a provision for automatic cut-in on low pressure.
The use of a waste gas with low-energy content or with unusual burning characteristics should be avoided.
182
This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
Parallel instrumentation for pressure reduction is frequently justifiable.
The flare fuel system should be carefully checked to ensure that hydrates cannot present a problem.
FO
Because of small lines, long exposed runs, large vertical rises up the stack, and pressure reductions, use of a
liquid knockout pot or scrubber after the last pressure reduction is frequently warranted.
If at all feasible in terms of distance, relative location, and cost, a low-pressure alarm should be installed on the
fuel supply after the last regulator or control valve so that operators are warned of any loss of fuel to the pilots.
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G.7.3 Pilot Monitoring (5.7.7.3)
G.7.3.1 Introduction
Several methods of pilot monitoring are available, including thermocouples installed within the pilot head; ionization
monitoring within the pilot head; and remote acoustic, infrared, or optical monitors.
G.7.3.2 Design Considerations
Experience has shown that thermocouples can fail due to high-temperature exposure.
The user should base the pilot-monitoring system on relevant experience for the specific application.
[8]
Further details concerning the design of pilot-monitoring systems can be found in API 537 .
G.7.4 Retractable Thermocouples (new)
G.7.4.1 Introduction
Retractable thermocouples are used on elevated flares and ground flares to allow maintenance / replacement of
pilot thermocouples while the flare remains in service. On elevated flares the retractable thermocouples are
designed to retract to grade at the base of the flare stack. On ground flares, the retractable thermocouples are
designed to retract to a location at grade outside the radiation fence or combustion chamber of the ground flare.
Repair / replacement of the pilot thermocouples is one of the more frequent maintenance items on a flare system.
It is common for pilot thermocouples to require replacement more frequently than flare systems are shut down for
maintenance / plant turnaround. Installing retractable thermocouples allows replacement of failed thermocouples
at any time without shutdown of the flare system.
G.7.4.2 Design Considerations
Retractable thermocouple systems typically consist of a long continuous sheathed thermocouple mounted in
stainless steel guide tubing that extends from grade to the thermowell on the pilot. A separate tubing run is
required for each thermocouple on each pilot. It is important that the stainless steel tubing system is properly
supported to ensure minimal bends in the tubing run. The inside surface of the tubing should be a smooth finish to
ensure proper sliding of the sheath in and out of the tubing. The thermocouple sheath material is typically 310SS
sheathing. The tubing material is typical 316SS.
It is important that the retractable thermocouple system and pilot be designed to ensure reliable seating of the
thermocouple in the pilot thermowell when a new thermocouple is installed. When the thermocouple sheath is
retracted, it is typically coiled as it retracts. The sheath should be as straight as possible when inserted into the
tubing to install the retractable thermocouple. A mechanical straightener device may be considered to ensure that
the sheath is straight when inserted.
For tall flare stacks above 76 m (250 ft) overall height, care should be taken to ensure minimal bending of the
guide tubing to allow the thermocouple sheathing to move freely within the tubing for the long vertical run required.
For tall flare stacks or long runs of retractable thermocouples, a mechanical pusher assembly may be considered
to ease the installation of the thermocouple.
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184
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publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman
of the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
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This document is under review as revision to an API Standard; it is under consideration within an API technical committee but has not received all approvals required for
publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of
the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
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publication. This document shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman
of the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.
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[42]
BRZUSTOWSKI, T.A. and SOMMER, E.C. Jr., Predicting Radiant Heating from Flares, Proceedings-Division of
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